Individuals of two subspecies of Ophryotrocha puerilis (Polychaeta; Dorvilleidae) were collected from differing thermal regimes, and cultures were maintained for over a year. Despite common rearing, the two subspecies show substantial differences in somatic growth rate. At 15°C, the warm-water subspecies grew more slowly, while at 20°C growth for the two subspecies was not significantly different. At 24°C, the warm-water subspecies grew more rapidly and suffered substantially less mortality than the northern subspecies. These results conform to a model predicting genetic differentiation of metabolic efficiency, leading to differences in growth efficiency among populations adapting to thermally differentiated habitats. The problems faced by the cold-water subspecies at 24°C conforms to expectations based upon natural habitat temperatures.