Filarioid nematodes of the genus Litomosoides occur in the abdominal and (or) thoracic cavities of marsupials, rodents, and bats of the Nearctic and Neotropical regions. In this study, the phylogenetic relationships among these nematodes were estimated with a parsimony analysis of morphological characters derived from species descriptions. This nonweighted analysis produced 20 shortest trees. The monophyly of the genus was not supported in that Litomosoides thomomydis and Litomosoides westi failed to group with the other members of the genus. When these 2 taxa (parasites of pocket gophers) were excluded, monophyly of Litomosoides was supported by 2 synapomorphies (structure of the walls and general shape of the stoma); however, ancestor–descendant relationships among the species in the genus were not well resolved. A posteriori reweighting of the characters produced a single tree, different from all 20 most parsimonious trees. Alternative host–parasite evolutionary models were tested against these results supporting the process of host switching as being most important in forming the patterns of mammal–nematode associations that have been detected in this group of nematodes.