Recent research has suggested that crayfish and their branchiobdellid (Annelida) symbionts are engaged in a cleaning symbiosis.  These experiments, however, were conducted in laboratory settings.  We evaluated the effect of a branchiobdellid worm on crayfish growth and mortality in a field experiment conducted in the New River, NC, during 2008.  Crayfish were randomly assigned to one of three treatments: 0, 4 or 12 worms.  A fourth treatment, no crayfish, served as a control for crayfish effects on sediment accumulation.  There was a significant effect of worm treatment on crayfish growth with crayfish in the 4 worm treatment exhibiting the highest growth.  Crayfish in the 12 worm treatment exhibited less growth than the 0 worm controls suggesting that at high worm densities the interaction had shifted from a mutualism to a weak parasitism.  Crayfish survivorship was higher in the two worm treatments.  Worm presence on crayfish also indirectly affected sediment accumulation on standardized substrata by apparently influencing crayfish behavior.  We have demonstrated that mutualisms like the cleaning symbiosis involving crayfish and branchiobdellid worms do not necessarily have fixed outcomes and that the mutualism-predation continuum does apply to cleaning symbioses, at least those in which the cleaner lives on the client.