Mark-recapture studies of some aspects of the biology of the cavernicolous crayfish, Orconectes inermis testii (Hay), were conducted from December, 1969 to March, 1970, in Mayfield’s Cave, Monroe County, Indiana. Population size was estimated to be 66 +/- 9 (95% C.L.) for the 300 m study area, but because of the small sample size, this is undoubtedly a deflated value. Size of animals, expressed as total length, indicates that the population was comprised primarily of adults. Seventy-four percent of the marked crayfish moved no more than 10.5 m away (total upstream and downstream distances) from the tagging site. Hence, this species appears to restrict its activities to a specific area (“home range”) of up to 10.5 m of stream passage. Form I males travelled greater distances than did Form II (15.1 and 3.0 m, respectively), possibly in search of mates. Adult females moved less than juveniles, and males appeared to move greater distances than females (means of 12.9 and 5.9 m, respectively). Upstream movements were more commonly observed than downstream (mainly Form I males), indicating a possible restocking mechanism following floods. Distances travelled were not rebated to the size of individuals or to elapsed time.