The 49 species of thoracican barnacles (the goose and acorn barnacles) that are known to occur in the northern Gulf of Mexico are differentiated by a taxonomic key based on external characters, with extensive illustrations and a glossary. The guide also includes an annotated list of those species and three systematically questionable ones, with areal range, bathymetry, substratum type and other data; a list of 16 species that have been found in the southern gulf of Mexico but not in the nothern Gulf; directions for collecting and examining specimens; and a list of references especially useful for further study of particular groups.