Carpenter bee females (genus Xylocopa) enter only their own nests. They can be found in the same burrow over a period of months, regardless of whether they arc living singly or aggregated in close vicinity. The aim of this study was to investigate and to clarify which sensory systems are responsible for the recognition of the nest. Females of three different African species were tested in their natural habitat. Following alteration in the immediate visual surroundings of either the entrance or the entire dwelling stem, bees behaved as they did in undisturbed situations and entered their nests. Furthermore, a relative dislocation of the entrance within the dwelling stem or a displacement of the whole stem within a restricted range had no effect on the recognition of the burrow. When the entrance was plugged by foam rubber the bees landed only after extended searching. When the entrance was closed by a strip of scotch tape the bees searched for several minutes, departed, then returned and searched again with the same result. The bees showed no indications that they recognized their burrow. These results strongly support the conclusion that the bees recognized their burrows primarily or exclusively by olfactory cues, and, furthermore, that individual odors were involved. The possible contributions of different sensory systems for the recognition of the nest and the possible biological function of the use of olfactory cues are discussed.