The crayfishes Orconectes peruncus and O. quadruncus are endemic to the upper St. Francis River drainage in southeastern Missouri, and were classified as threatened in a recent analysis of the conservation status of crayfishes in the United States and Canada. Their distributions are not documented and little is known about their ecologies. In addition, recent collections suggested that the introduced crayfish O. hylas may be displacing O. quadruncus from its type locality, Stouts Creek (Madison and Iron counties). The purpose of our study was to assess and compare the distributions and selected aspects of the life histories and habitat associations of these crayfishes. Sampling at 57 sites on 30 streams in the upper St. Francis River drainage from October 1996 through March 1998 showed that O. peruncus was more widespread and more abundant than O. quadruncus. The two species were sympatric in only three streams. Patterns of reproductive events in the species' annual life cycles were similar, although O. peruncus showed signs of mating activity a month earlier than O. quadruncus. The crayfishes showed similar habitat associations. Orconectes quadruncus has been largely displaced from Stouts Creek by the introduced O. hylas. Orconectes hylas also potentially threatens O. peruncus in Big Creek, warranting further study.