The objective of this study was to examine the regeneration capacity of the spionid polychaete Marenzelleria viridis from Long Island, New York. In the field,~7% of the worms exhibited regeneration of the anterior end. In the laboratory, worms were ablated at the 10th-50th chaetiger and their regeneration documented. Anterior morphogenesis was similar to that previously reported for spionids, with wound healing, blastema formation, differentiation of segments, and formation of feeding and sensory structures (mouth, palps, nuchal organs) occurring within 14 d. Unlike in some spionids, the segments do not appear to all form simultaneously from the blastema; rather, external differentiation of segments was observed from posterior to anterior on the regenerate. The number of segments replaced was equal to the number ablated for up to 10 segments. A maximum of 17 segments were replaced when 20-30 chaetigers were ablated, and the number replaced decreased to 14 when 40-50 chaetigers were ablated. Survival and normal growth of the worms decreased with more chaetigers ablated; a significantly higher number of worms died or grew abnormally with ≥30 chaetigers ablated, compared to worms with ≤20 chaetigers ablated. Members of M. viridis could be valuable model organisms in the study of the cellular mechanisms involved in regeneration, and further research on regeneration in the field should be completed.Additional key words: marine, northwest Atlantic, Polychaeta, regenerate, sublethal predation Invertebrate Biology 135(4): 357-369.