Asymmetry in mutual predation can be important in species replacement between native and invasive species. Mutual predation between the endangered Japanese native crayfish species Cambaroides japonicus and the North American invasive crayfish Pacifastacus leniusculus was studied to assess the potential impact of P. leniusculus on C. japonicus. We made laboratory experiments to examine mutual predation (i) between individuals in pairs of the same and different species and (ii) among a group comprising single and mixed species. We also made field surveys to clarify the frequency of cannibalism of the two species. In laboratory experiments, the frequency of cannibalism was low by both two species, but predation by P. leniusculus on C. japonicus was severe and most P. leniusculus survived both in different species pairs and in mixed species groups. In field surveys, remains of the two crayfish species, which may have been caused by predation by the same species, i.e., cannibalism, were scanty. The results suggest that asymmetry in mutual predation can be an important cause of species replacement of the native C. japonicus by the invasive P. leniusculus in the field.