Stable carbon and nitrogen isotopes have been used to characterise sewage discharge and effects on seaweed(Ulva lactucaL.), blue mussels (Mytilus galloprovinialis), and limpets(Cellana denticulata)from Moa Point Bay, New Zealand. The nitrogen (15N/14N) ratio is shown to be a good tracer of sewage pollution inUlva lactucaand associated grazers (Cellana denticulata)as a result of the increased contribution of urea and ammonia to the surrounding marine environment from the sewage outfall. The carbon (13C/12C) ratio is suggested as a more appropriate sewage tracer for mussels(Mytilus galloprovinialis), which filter feed the particulate organic matter from the effluent. Lower carbon : nitrogen ratios were found inU. lactucasampled from the outfall region, compared to uncontaminated control sites, however carbon: nitrogen ratios do not vary significantly amongst shellfish species.