Our state of knowledge of the polychaete fauna of the entire Gulf of Mexico is generally very poor. The polychaetes are a difficult group with which to become familiar and consequently are neglected by most marine biologists, or are sent off to be identified by the few competent, but overtaxed, polychaete specialists. Hopefully, the key will make it easier for students in marine biology to identify polychaetes themselves. Not all the polychaetes in this key have been reported from the northwestern Gulf of Mexico, but all species inhabit the Gulf either in the Carolinean or Caribbean Zoogeographic Provinces, which merge along the lower Texas coast.