The parapodia of two species of polynoid polychaetes, Halosydna brevisetosa and H. johnsoni, were amputated and allowed to regenerate under varying temperature conditions to test the validity of the structure of the neurosetae as a systematic character. H. brevisetosa has entire neurosetae and inhabits water 5 to 6°C colder than H. jolmsoni. This latter species has bifid neurosetae. Trends toward bifid setae were noted when regeneration of parapodia occurred at warm temperatures and entire setae developed at cold temperatures. The authors concluded that H. jolmsoni is a synonym of H. brevisetosa.