Energy sources for the crayfishes Orconectes luteus and O. punctimanus in the Jacks Fork River, Missouri, were quantified using stable isotopes (??13C and ??15N) and gut-content analysis. A dual-isotope mixing model indicated that about two thirds of crayfish production originated from allochthonous carbon sources, and 30-50% of crayfish production was derived from direct consumption of animal matter. Young-of-the-year crayfishes and adult O. luteus were more carnivorous than the larger adult O. punctimanus. Contributions of energy sources determined using the stable isotope mixing model were similar to results of gut-content analysis corrected for differential assimilation of dietary components. Proportions of crayfish production attributed to animal matter by these two methods were four to five times greater than estimates derived from uncorrected gut-content analysis. Unadjusted gut-content analysis overestimated the percentage of crayfish production from feeding directly on detritus. Production-based calculations of crayfish food consumption rates indicated that crayfishes were the dominant consumers of benthic invertebrates, detritus, and algae and may strongly influence lower trophic levels, organic matter processing, and energy flow in this system.