Several methods for permanently marking cavernicolous crayfishes were investigated prior to initiation of field work in Pless Cave, Lawrence County, Indiana. Internally injected ink complemented with external “painting” proved to be a most satisfactory tagging procedure. During the two-year study period 211 individuals of the troglobitic crayfish Orconectes inermis inermis Cope were marked; 96 tagged individuals were recaptured at least once, a 46% recapture rate. The population size was estimated to be 1586 +/- 79 (95% C.L.) over the 540 m subterranean stream study area and remained relatively stable during the period of 1970 to 1972. The home range of male crayfishes is as high as 20 m and extends up to 23 m for females, although maximum distances travelled by individuals of both sexes greatly exceed these values. Small individuals of both sexes are displaced downstream whereas larger crayfish show distinct upstream movement. If all movement data are pooled, both sexes exhibit a net downstream movement. The downstream movement of crayfish is heavily influenced by flooding.