Some aspects of the biology of a population of the hermit crab,Cestopagurus timidus,collected from aPosidonia oceanicabed off the Ligurian coast, were investigated also in relation to housing gastropod shells. The population studied was composed by specimens with a carapace length not exceeding 3.6 mm and a sex‐ratio close to 1:1. Ovigerous females were present from March to September. The young, small‐sized specimens occupiedBittiumshells, whilst the medium and large‐sized ones preferred shells of other genera, particularlyCantharus.Interspecific and intersexual competitions for the shell were not apparent; it is possible that similar‐sized specimens may fight for the preferred shells. The few ovigerous females observed occupied shells adequate to their size. Among the different shell parameters examined, linear dimensions appeared to be valid and reliable for evaluating the shell suitability for the hermit crab.