The genus Southwellina is composed of 3 described species, i.e., S. hispida (the type species), S. dimorpha, and S. macracanthus. All 3 are endoparasites of fish-eating birds that have worldwide distributions. Morphologically, the genus is characterized by possessing a short and compact trunk, 2 fields of spines in the anterior region of the trunk (in at least 1 sex), a short cylindrical proboscis (sometimes with a swollen region armed with numerous longitudinal rows of hooks), a double-walled proboscis receptacle, and 4 tubular cement glands in males. In the current study, specimens identified as S. dimorpha were collected from Eudocimus albus (white ibis), the type host from the Gulf of Mexico. Sequences of 2 nuclear genes (small subunit [SSU] and long subunit [LSU] ribosomal DNA) and 1 mitochondrial gene (cytochrome c oxidase subunit 1 [cox 1]) of S. dimorpha and S. hispida were obtained and used to reconstruct the phylogenetic relationships of both species with respect to published sequences of 11 species representing 6 genera of Polymorphidae. Maximum parsimony (MP) and maximum likelihood (ML) analyses of the concatenated data set (SSU LSU cox 1) were identical in depicting Southwellina as paraphyletic, indicating that the genus should be revised. The MP and ML trees identified S. hispida as a sister to Polymorphus brevis, whereas S. dimorpha was a sister to Hexaglandula corynosoma. Morphologically, S. dimorpha is distinct from H. corynosoma, which is characterized by a short trunk with 1 field of spines in the anterior part of the trunk in both genders, and males with 6 tubular cement glands. The genetic divergence estimated from a concatenated data set between 2 isolates of S. hispida and S. dimorpha ranged from 10.7 to 11.0%. This range of genetic divergence is similar to that found among other genera of Polymorphidae, which extends from 6.0 to 12.0%. Southwellina dimorpha differs from S. hispida in the shape of the proboscis and the presence of 1 field of spines (S. dimorpha) versus 2 fields (S. hispida) on the anterior region of the trunk in females. Based on the phylogenetic position of S. dimorpha within Polymorphidae, coupled with levels of genetic divergence and, more importantly, the morphological and ecological (host specificity) differences, we propose the erection of a new genus to accommodate S. dimorpha.