A 3 Ο« 2 factorial experiment was conducted to evaluate the influence of food ration on observed toxicity in Neanthes arenaceodentata. Worms were fed one of three food rations: 0.25Ο«, 1.0Ο«, and 6.0Ο«. The 1.0Ο« ration was composed of 4 mg TetraMarin/worm/week and 2 mg alfalfa/worm/week. Worms in each food ration were exposed to one of two sediments: a clean control sediment collected from Sequim Bay, Washington, USA (SC) or a contaminated sediment collected from Black Rock Harbor near Bridgeport, Connecticut, USA (BRH) that was diluted with SC such that the final concentration of contaminated sediment was 12%. Survival and growth were monitored every 2 weeks for 8 weeks. Sediment treatment and food ration had strong effects on survival and growth patterns in N. arenaceodentata. Most of the mortality within any given treatment combination occurred during the first 2 weeks of the experiment. This decline in survival was greatest for worms exposed to BRH. Survival was significantly lower for worms in BRH compared to SC at the 0.25Ο« and 1.0Ο« ration levels. Survival was not reduced for worms in BRH compared to SC at the 6.0Ο« ration level. Worm size during the course of the experiment was positively related to ration level. Significantly reduced size in BRH worms was observed at each monitoring period within the 0.25Ο« and 1.0Ο« rations, but only at weeks 6 and 8 for the 6.0Ο« ration level. The percent difference in size between worms exposed to BRH and SC, when those worms were fed the 0.25Ο« or 1.0Ο« rations, ranged between 55 and 66%. The percent difference in size observed between BRH and SC worms fed the 6.0Ο« ration was much smaller (9-17%). The results of this study demonstrate that food ration has a strong influence on observed toxicity in N. arenaceodentata. The importance of food ration effects on toxicity must be considered while designing chronic sublethal bioassays. To ensure the relevance of bioassay results, the food ration used during a chronic bioassay should produce a pattern of growth in control animals that is similar to patterns of growth observed in nature.