The effect of shelter on the salinity preference behaviour of red and green colour forms of the shore crabCarcinus maenas(Crustacea: Decapoda) was investigated in the laboratory and the field. Red colour forms which are in a state of prolonged intermoult are less tolerant of low salinity than green colour forms and are absent from estuaries. In choice chamber experiments green crabs remained longer at low salinity if a shelter was available, but salinity preference behaviour of red crabs was unaffected by the presence of shelters. In an estuary the upper limit of distribution towards freshwater of green crabs was higher where shelters were made available. These results are discussed in relation to distributional differences and retention within the estuary.