Populations of harpacticoid copepods belonging to the family Phyllognathopodidae found on most continents are usually assignable to the taxon of Phyllognathopus viguieri, as it is presently diagnosed, on the basis of their morphological characters. This taxon has been considered as cosmopolitan since the unifying revision of Lang in 1948. Here we reveal that Phyllognathopus viguieri is not a single biological species. Cross-breeding experiments with individuals from five different populations of Phyllognathopus viguieri obtained from Germany, Maryland and Florida in the USA, Jamaica, and Brazil indicated that two of these mutually distant populations were reproductively isolated from each other but crossed easily with others, whereas the other populations did not interbreed, with the exception of the two populations from the USA. From these results we conclude that Phyllognathopus viguieri is not a single cosmopolitan biological species. The high proportion of mutually non-reproducing populations identified in the five populations examined in this study indicate that further analysis of the micromorphology and genetic composition of populations over the entire putative range of Phyllognathopus viguieri will be necessary to clarify their interrelationships.