Symbiotic interactions can affect populations, communities, and ecosystems by creating novel flows of material and energy in food webs. However, there is considerable variability in host specificity among symbionts. We examined the host specificity of an ectosymbiotic annelid worm (Annelida:Clitellata:Branchiobdellidae) on 2 sympatric species of host crayfishes. In host-selection experiments, the branchiobdellid Cambarincola ingens colonized the crayfish Cambarus chasmodactylus 2.5x more frequently than similarly sized Orconectes cristavarius even though O. cristavarius was 2 to 4x more abundant in the field. Infestation intensity–body size regressions for Cambarincola in the New River reflected host-selection experiment results; the slopes for C. chasmodactylus were up to 8x steeper than those for O. cristavarius. In addition, 96% of C. chasmodactylus had at least one attached Cambarincola compared to only 36% for O. cristavarius. There are many possible explanations for the observed host preference, including ratio-dependent selection and differences in host behaviors. However, the most likely explanations for host selection in Cambarincola seem to be differences in cost-benefit relationships or evolved direct responses to host identity.