Bradford Dillman was originally unhappy with his character's 2D nature, and asked writer John Sayles why his character was so thin. Sayles responded that Roger Corman regularly did not use good actors in his film, so he deliberately didn't elaborate on characters. But since Dillman was a "real" actor, he was more than happy to enhance his character's depth.

Universal Studios attempted to sue New World for spoofing Jaws (1975). However, Steven Spielberg saw the movie in advance and loved it. After that, Universal dropped the lawsuit.

The piranha were done by attaching rubber puppet fish to sticks.

A waitress from the Holiday Inn where the director and crew were staying stood in for Heather Menzies during the topless shots. Menzies was concerned that her husband might not approve of the nude scene.

The extras were all paid $5 a day and given a box lunch.

Executive producer Roger Corman called this film "my homage to Jaws (1975)."

This film was reported to have been shot in 30 days for $660,000.

This movie was one of the first efforts for effects and makeup artists Rob Bottin and Phil Tippett.

Rick Baker was originally tapped to provide the make-up effects. He recommended 17-year-old Rob Bottin instead.

Barbara Steele's role was originally written for a man.

The score cost $10,000 dollars.

John Sayles used the profits from the film to fund his own projects. It was also his scriptwriting debut.

The Aquarena Springs and Resort theme park, in San Marcos, TX, opened in 1951. It was later purchased by Southwest Texas State University, now Texas State University - San Marcos. The amusement park shut down in 1996. The facility is now known as Aquarena Center, an environmental learning center.

Steven Spielberg described this film as "the best of the Jaws (1975) rip-offs." He and Joe Dante later collaborated on Twilight Zone: The Movie (1983).

Originally, Maggie counted to 300 instead of 100 at the climax. This was probably changed because in all likelihood, Paul would have drowned after being underwater for that long.

The novelization also fills in some trivia about Maggie. When her boyfriend walked out on her, she hired a private detective to find him. She got so hooked on the process she decided to become one herself. By the time she caught up with her ex, she wasn't interested anymore in why he left.

Peter Fonda was originally offered the role of Paul Grogan, but turned it down.

The film had some documented production problems, including last-minute cast changes, underwater cameras that kept breaking down, union woes, and unusable second unit footage. It still became one of New World Pictures' biggest hits.

The video game that Maggie (Heather Menzies-Urich) is playing toward the beginning of the movie is "Jaws".

When the guard at the dam is watching a commercial promoting the resort at Lost River Lake, originally some of Dumont's campers were part of the promotion, but this was cut from the movie. Another scene cut was Dumont accidentally appeared in the background of the commercial, ruining the shot. He was ordered off the premises by Buck Gardener.

Barry Brown's last role in a theatrical feature.

The special effects budget was $50,000.

The film was designed to jump on the huge success of Jaws (1975). Roger Corman was known for producing films that were veiled imitations of more successful movies. But it still took a further three years to raise enough money to make the film. By which time Jaws 2 (1978) had already been made.

Eric Braeden was originally cast as Paul Grogan, but he pulled out to pursue another project. He had already shot some underwater swimming footage which is used in the film.

Although implied by the film, the novelization confirms that Paul and Maggie slept together while Hoak is their prisoner.

The novelization fills in some of the background about Paul's character. His wife left him which is why he's such a hard drinker. The only explanation given is that she couldn't stand the sight of him. He doesn't write or try to contact his daughter, something Laura the camp councilor resented him for. She later changed her mind about Paul after saving (some of) the kids from piranha. Including his daughter. And the main reason Dumont thinks he's drunk over the phone is because during one visit to the camp, Paul drunkenly threw Dumont in the river when he told him to leave.

The main leads Heather Menzies-Urich & Bradford Dillman died within 4 weeks of each other over the December 2017/January 2018 period.

One of a cycle of 1980s and mid-late 1970s movies that got made after the box-office success of Jaws (1975). The films include that movie's three sequels, Jaws 2 (1978), Jaws 3-D (1983), and Jaws: The Revenge (1987), as well as Orca (1977), Piranha (1978), Tentacles (1977), Killer Fish (1979), Barracuda (1978), Tintorera... Bloody Waters (1977), Blood Beach (1980), Piranha Part Two: The Spawning (1981), L'ultimo squalo (1981), Up from the Depths (1979), Monster (Humanoids from the Deep) (1980), L'isola degli uomini pesce (1979), Devouring Waves (1984) and Mako: The Jaws of Death (1976).

According to Joe Dante in the DVD commentary originally released in 2004, the noise that the piranhas make underwater are the sounds of dental drills. While he didn't go into anymore detail than that, they could the put the edge of the drills into water and an underwater mic captured the sounds when they were turned on. Another possibility is the drills were turned on above water and recorded from underwater.

The breed of piranha is Brazilian in the novelization.

Entertainment Weekly ranked this Number Nine on their "Guilty Pleasures: Testosterone Edition" list in their March 30, 2007 issue.

In one if the scenes a girl is seen reading Moby Dick, this story is also about a marine animal attacking people.

Belinda Balaski's death scene was shot in a pool. Thirty fake piranha were attached to Balaski's body with gaffer's tape, and a bunch of crew members pulled her into the deep end of the pool with ropes to make it look like her character was sinking into the water. Roger Corman demanded a reshoot because he thought the first version didn't have enough blood.

A "bobbing corpse" that appears in the climactic piranha attack was modeled after Rob Bottin.

The boy on the canoe originally didn't emerge unscathed like he does in the film. He suffered severe lacerations on his arms but this doesn't appear in the film. He is however in a state of shock because of his father's death. He even thinks Hoak is his father, which is why he keeps calling him Daddy when Paul and Maggie dump his body overboard.

In the novelization, Brandy dies as well when she jumps in the water after the piranha to avenge Jack. Since she isn't seen again after Jack's body is found, that scene was probably cut from the film.