1. Estimates of osmotic water permeability suggest that Libinia emarginata [166 µl/(g·hr·osmol)] is less permeable to water than Pugettia producta [338 µl/ (g·hr·osmol)]. Data on tracer water exchange supports this conclusion and the differences in apparent permeability can be correlated with differences in habitat. 2. Short-term changes in D2O water exchange were examined in specimens of Carcinus maenas and Libinia emarginata. When these crabs were transferred from 100 to 80% sea water, there was an initial reduction in k in (1/hr), the hourly water exchange fraction, of from 1.9 to 1.4 and from 5.8 to 4.3 in Carcinus and Libinia, respectively. In both crabs, the initial response to the dilute medium is similar; however, in Libinia, the reduction in k is transient while in Carcinus it is a long-term response. 3. Estimates of osmotic (Posm*) and diffusive (Pd*) water permeabilities for Carcinus, Libinia and Pugettia indicate that the ratio of Posm*/Pd* is about 2, which suggests that unstirred layers could have a major influence on tracer water movement in all of these crabs. It is proposed that the initial changes in k, which occur during adaptation to a dilute medium, are at least partly the result of an increase in thickness of the unstirred layers surrounding the gills, caused by a reduction in the flow of medium through the gill chamber.