The sites where haddock spawn and the factors that bring the fish together to spawn at particular locations are not well known. We have located haddock in the sea by listening for the sounds this species makes during its reproductive behaviour. The characteristics of the sounds made by haddock were first examined in the aquarium. Listening was then carried out in a Norwegian fjord, where fishers had reported spawning haddock. Long sequences of repeated knocks were heard in the fjord, similar to the display sounds recorded during stereotyped reproductive behaviour in the aquarium. Rapidly repeated knocks and humming were also heard, confirming that fish were engaging in courtship. At night the sounds merged into a continuous low frequency rumble, suggesting that many haddock were present, producing sounds simultaneously. Listening for haddock sounds provides a reliable, non-invasive way of locating aggregations of spawning haddock in the sea, allowing closer definition of the spawning areas. Both fishing and exposure of haddock to man-made noise at the critical time of spawning may have deleterious effects upon reproductive success.