Six species and subspecies of crayfishes and four species of entocytherid ostracods are known to inhabit the subterranean streams of southern Indiana. Cambarus (E.) Iaevis (troglophile) appears to be the most widely distributed crayfish and occurs in both karst areas within the State. The troglobite, Orconectes inermis (2 subspecies), is restricted to the larger karst area in solution cavities of Mississippian carbonate rocks. The remaining crayfishes, Orconectes immunis, Orconectes propinquus and Orconectes sloanii, are not common inhabitants of cave waters and are probably trogloxenes. All of the crayfishes except O. sloanii were found to host at least one species of ostracod. From data presented, Sagittocythere barri might be expected to be found commonly in association with Orconectes inermis, Donnaldsoncythere donnaldsonensis, Uncinocythere xania and Dactylocythere susanae, however, are more commonly associated with C. (E) laevis, indicating a near host-specific relationship among these taxa. Whether these are host-specific associations or ones imposed by certain ecological parameters will require additional investigations. Although a fair understanding of the distribution of these crustaceans in the larger, Mississippian limestone belt has been obtained, additional field work on the perimeter of the spelean ranges of the several species will probably prove productive. Furthermore, considerable cave exploration and biospeleological surveys are needed in the Silurian-Devonian limestones of southeast Indiana before our knowledge of these crayfishes, entocytherids and other cave-dwelling species approaches that for the Mississippian karst of the State.