San Francisco, Washington and New York are using bluegills - also known as sunfish - to safeguard their public drinking water. A small number of fish are kept in tanks which are constantly filled with water from the municipal supply.
The computerised system registers changes in the fishes' vital signs and sends an alert when something is wrong.
Since 11 September 2001, the US government has taken the threat of attacks on water supplies seriously.
Under federal law, nearly all community water systems must be assessed for their vulnerability to terrorism - and water supplies are constantly monitored and tested for chemical and biological agents. "It's like an early warning system - it acts as another line of defence," said Bill Lawler, co-founder of Intelligent Automation Corporation, the San Diego-based company that makes the anti-terror apparatus.
Bluegills - a hardy species - are highly sensitive to a wide number of toxins. When they are exposed to such substances they experience the fish version of coughing, flexing their gills to expel unwanted particles.
At the first sign of stress in the fish, the computer system will send an alert by email, pager or mobile device, also known as "fish phones".
New York City's bluegills were put to work recently when the system caught traces of a diesel spill before any of the Department of Environmental Protection's other devices.
The bluegills do have limitations however. They cannot reliably detect germs and are no use against other sorts of attacks - the bombing of a water main, or computer hackers attacking the systems that control the flow of water.
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