This study investigated the effect of branchiobdellids (Annelida: Clitella) on crayfish growth and stamina. A 115-day lab investigation showed that the branchiobdellid Cambarincola fallax did not decrease the growth rates nor lower the stamina of its host crayfish Orconectes rusticus. Among crayfish originally given 15 worms, there was a significant, positive correlation between final host size (carapace length P<0.05 and wet weight P<0.01) and number of branchiobdellids found. No correlation existed between the number of molts observed and branchiobdellid number. These results support the hypotheses that C. fallax has a commensal association with O. rusticus and that host molting is a critical time for branchiobdellids.