Host specialization has contributed to the high diversity of laelapine mites associated with Neotropical rodents, but the lack of taxonomic development at the species-level has confounded study of the coevolutionary history of both host and ectoparasite groups. Morphometric comparisons of presumptive polyxenous laelapine species infesting a diverse assemblage of palustrine rodents in Paraguay clearly reveal that each host species is infested by a morphologically distinct mite population. The nominal taxa Laelaps manguinhosi, Gigantolaelaps goyanensis, and G. mattogrossensis may be composites of morphologically distinct but similar species with narrower host preferences. These results suggest that laelapine mites are primarily monoxenous, and that numerous currently unrecognized species may be discerned by standard morphometric techniques.