The shape of the dorsal vesical plate was used to determine the extent of morphological differentiation among populations of the water strider Aquarius remigis (Say) (Hemiptera-Heteroptera: Gerridae), and between species assigned to the A. remigis species group. Populations were sampled throughout North America and Mesoamerica, and included paratypes of A. remigoides Gallant & Fairbairn, and material collected from the type localities of A. amplus (Drake & Harris) and A. nyctalis (Drake & Hottes). If taxonomy is to be inferred from the shape of the dorsal vesical plate, as suggested by previous authors, several taxonomic issues need to be reconsidered. First, there was one species distributed widely across North America and deep into Mexico that is very likely to represent A. remigis, meaning that populations from the western USA previously assigned to ‘A. nyctalis’ should be synonymized with this species. Second, A. remigoides as currently defined also includes some populations of A. remigis, and is probably much less widely distributed in the south-eastern USA than previously thought. A hybridization zone in Pennsylvania between A. remigis and A. remigoides, as inferred from measurements of body size and allozyme electrophoresis, is not congruent with data from the male genitalia, which indicated a transition zone between the two forms in Virginia much further to the south. Finally, A. amplus was widespread in Mexico, reaching into Guatemala to the south and into Arizona to the north. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 381–398.