In predator–prey relationships such as those between crabs and their bivalve prey, interference competition is a topic of intense investigation as it can have profound consequences on the dynamics of both predator and prey populations. However in laboratory experiments – also those on crab–bivalve systems – workers never adequately disentangled interference competition from exploitative competition, as prey depletion was never compensated. Hitherto, experimental studies on crab–bivalve systems lack direct behavioural observations and have provided only indirect and thus inconclusive evidence of interference competition. We studied interference competition in adult male shore crabs Carcinus maenas that foraged on blue mussels Mytilus edulis. We developed a novel type of experimental tank to replenish each consumed mussel, and thus to keep prey levels constant. We conducted two experiments in which we varied number of crabs (1, 2, 4) and number of mussels (first experiment: 4, 8, 16, 32; second experiment: 8, 32, 128) and directly observed the foraging behaviour of crabs (foraging area=0.25 m2). In the first experiment, feeding rates decreased with increasing crab density only at mussel density 16 because both search time and time spent in agonistic interactions increased. At other mussel densities, variation in crab density did not affect feeding rates, possibly because of low statistical power and the narrow range of mussel densities offered. In the second experiment feeding rates decreased with increasing crab density because crabs spent more time in agonistic interactions and handling their prey. Feeding rates increased with increasing mussel density. Overall, crabs spent on average 14–18% of their foraging time in agonistic behaviours, while on three out of 64 occasions feeding rates decreased because mussels were stolen (kleptoparasitism). Concluding, we have shown that interference competition occurs in absence of prey depletion, while conducting direct behavioural observations aid to identify the behavioural processes that underlie interference competition.