The crayfish Orconectes eupunctus is endemic to the Spring and Eleven Point rivers of Arkansas and Missouri, and appears to have been displaced from a portion of its range by the recently introduced crayfish O. neglectus. Life history can be a factor in crayfish species displacements as earlier reproduction or rapid juvenile growth may provide size advantages in competitive interactions, while higher fecundity might permit an invading species to out reproduce a native. We conducted a comparative study to determine if life history differences exist between O. neglectus and O. eupunctus that could contribute to the displacement of O. eupunctus. Crayfish were collected at three sites (each species alone and both species together) monthly from Jul. 2005 through Dec. 2006 by kick-seining. Timing of reproductive events was recorded, fecundity was evaluated through egg counts, and juvenile growth rates were examined using length-frequency histograms. Sex ratios, length-weight relationships, and adult size structure were also documented. Timing of reproductive events was synchronous between species, with males molting to Form I in Sep.–Oct., females producing glair in Nov.–Dec., and oviposition in Mar.–Apr. Orconectes neglectus females carried significantly more eggs than O. eupunctus females, but a greater proportion of O. eupunctus females were ovigerous than O. neglectus. Juvenile crayfish of both species recruited to the population in May and achieved similar sizes by the end of the growing season. Orconectes eupunctus adults in the presence of O. neglectus were smaller than both O. neglectus adults and O. eupunctus isolated from the introduced species. Both species showed a typical annual pattern in sex ratios for Orconectes spp. crayfishes, with male-skewed sex ratios when females were ovigerous in the spring and female-skewed sex ratios in other seasons. The sex ratio of O. eupunctus in the presence of O. neglectus was male depleted relative to both O. neglectus and O. eupunctus alone. For both O. neglectus and O. eupunctus, male crayfish weighed more than female crayfish, but no significant differences existed between the species within the same sex. Results of this study will complement and inform further research into a potential mechanism for the apparent displacement of O. eupunctus by O. neglectus, and also help document the under-studied life histories of crayfishes endemic to the southeastern United States.