We investigated whether Thalamita crenata, a swimming crab found on the East African intertidal flats, uses landmarks to locate its refuges. We modified the visual panorama of an intertidal flat, using conspicuous movable objects, and conducted homing trials with the local population of swimming crabs. In the first set of trials, after being moved away from their dens, the crabs were immediately able to find other known dens, using direct shortcut paths. In the second set of trials we moved all the artificial landmarks surrounding a crab's den 5 m away and then displaced the crab itself. The crabs made initial navigational errors in accordance with the new position of the landmarks; this shows that they oriented themselves by remembering the landmarks. We then repositioned the landmarks and released the crabs far from the familiar area, on a similar flat. This time the crabs could rely only on the artificial landmarks; they used this information and reached the point where home should have been according to the landmarks. Thus, T. crenata showed good spatial knowledge, based on the storage of landmark memories. This orienting mechanism is much more flexible and complex than those of other crabs and is comparable to the route-based memory of honeybees, Apis mellifera. Copyright 2000 The Association for the Study of Animal Behaviour.