A sphaeriid clam, Sphaerium occidentale, a frequent inhabitant of temporary woodland ponds, was examined in the laboratory to learn its tolerances to 15 C, room temperature (average of 22 C) and 30 C at relative humidities (RH) of 94%, 77%, 33%, and 0%. It lost weight (from soft parts and fluids) rapidly at 30 C and at low humidities. At room temperature it survived for more than 5 days in 94% and 77% RH, losing less than 10% of body weight. At 33% and 0% RH, 30% and 50% of body weight, respectively, was lost over this same period. A maximum of 34.2% loss of weight was sustained without death; however, some succumbed upon loss of 12%. Laboratory tolerances exhibited by S. occidentale compared with observed physical conditions of the natural habitat suggest that these conditions seldom exceed what the undisturbed sphaeriid can withstand. Limited observations were made which suggest that this species may enter a condition of aestivation in low humidities.