Ergasilid copepods represent one of the commonest groups of fish parasites in Brazil. Within Ergasilidae, three genera share a peculiar latching mechanism on the antenna that completely encircles the gill filament, one of which is Acusicola Cressey, 1970. During a survey of estuarine fish from the Brazilian Amazon Coast, a new species of Acusicola was found on the gills of the largescale foureyes Anableps anableps (Linnaeus, 1758) (Actinopterygii: Anablepidae) in São Marcos Bay, State of Maranhão. Acusicola rochai n. sp. can be distinguished from its closest congeners mainly by three protrusions on the dorsal surface of third and fourth pedigerous somites, and by smooth interpodal plates. This work is the first report of a parasitic copepod infesting a fish from Anablepidae and, consequently, the host An. anableps. The existing dichotomous key proposed for the genus Acusicola includes only ten species, excluding the eight species subsequently described. Therefore, in the present work, a new dichotomous key is provided based on reliable and well-documented features.
