The phylogenetic relationships and historical biogeography of species of Sciadicleithrum parasitizing Neotropical cichlid fishes were studied. Hypotheses were: Did the presence of Sciadicleithrum species in southeast Mexico (SM) and Central America (CA) result from early dispersal of cichlid species from South America (SA) to the north via the Antilles during the Tertiary (24 million years ago [mya]), or did it occur after emergence of the Panama Isthmus in the Pleistocene (2–5 mya)? The Sciadicleithrum phylogeny was based on 19 morphological transformation series, with species of Gussevia and Cichlidogyrus as outgroups. The most parsimonious cladogram had a 40% consistency index, with one clade including all the species of Sciadicleithrum from SA and all but one of those from CA and another with all the Sciadicleithrum species from SM and S. maculicaudae from CA. Results support the late dispersal hypothesis following emergence of the Panama Isthmus. Fifteen species of Sciadicleithrum parasitize 13 cichlid species from SA and CA. In contrast, only 4 species of Sciadicleithrum infect 14 cichlid species from SM. Parasite speciation appears to lag behind host speciation, with 2 equally possible explanations: loss of parasite species and host switching.

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