American lobsters (Homarus americanus) will on rare occasions produce sounds by vibrating their dorsal carapace. Although this behavior can be elicited in the laboratory by handling lobsters, the stimulus that triggers the production of sounds in the lobster's natural habitat is not known. We investigated the influence of two fish that are known to prey on lobsters, cod (Gadus morhua) and striped bass (Morone saxatilis), on the production of sounds by American lobsters. In addition, we examined the response of the same fish to the sounds the lobsters produced. Although solitary lobsters spontaneously produced sounds at a low rate of 1.2 ± 0.23 sound events per 30 min, the presence of a single cod or striped bass led to an increase in the rate of sound production (cod: 51.1 ± 13.1 events per 30 min; striped bass: 17.0 ± 7.0 events per 30 min). Most (74.6 ± 6.6%) of the 292 sound events recorded occurred when a fish came within 0.5 m of a lobster, but a fish did not have to come into contact with a lobster to elicit sounds. Immediately following the production of a sound by a lobster, fish turned and swam away significantly faster than when they encountered a lobster that did not make a sound. Moreover, after striped bass (but not cod) experienced a number of these sound events, they subsequently tended to avoid swimming close to the lobsters. These data, taken together, suggest that sound production by American lobsters may serve to deter potential fish predators.