Fourteen species of Stenelmis from North America were studied: S. beameri, S. bicarinata, S. crenata, S. exigua, S. gammoni, S. grossa, S. mera, S. parva, S. sandersoni, S. sexlineata, S. vittipennis, and three undescribed species. This study was concerned with evaluating characters for separation of the species. The last larval instar was selected for interspecific comparison due to increasing ornamentation during larval development. The external anatomy is described in detail for the genus. Forty-four characters are used in diagnostic descriptions for the 14 species. A key for identification of these species is included. Numerical analysis of characters shows four main groups of characters: (1) most sclerite measurements; (2) most tubercle counts; (3) the prosternal tubercle count; and (4) the cervical sclerite length. Numerical analysis of the species indicates phenetic and phylogenetic relationships. In both sets of relationships, three clusters of species are found: (1) S. parva and S. gammoni; (2) S. sandersoni; and (3) the remaining 11 species. Information from this study lends support for or disagrees with various proposed cases of synonymy based on adult morphology.