Roger Corman produced this classic directed by Joe Dante.
Heather Menzies is a young woman working for a tracer company looking for two missing young people. She tracks them to a mysterious military base that has long been abandoned. Along the way she runs into a man living in a mountain cabin, played by Bradford Dillman. He accompanies her to the base and she accidentally drains a pool that turns out to be populated with a mutant strain of Piranha.
Living at the base is a scientist played by Kevin McCarthy who tries to stop them, but to no avail. The creatures proceed to kill many people as they make their way into the fresh water system. They are headed into a summer camp an finally into the ocean.
Dillman and Menzies desperately try and get people to believe them, but as usual nobody does. Dante does his usual great job directing and having fun with the story. It never slows down and it has a cast to die for. Besides the above mentioned people this also stars Barbara Steele, Bruce Gordon, Richard Deacon, Keenan Wynn, Dick Miller, Belinda Belaski, Melody Thomas and Paul Bartel.
This is a must see.