Abstract
Among the very rarest of Nearctic ants are three species of inquiline social parasites belonging to the genus Monomorium, namely Monomoriuminquilinum DuBois, Monomoriumpergandei (Emery), and Monomoriumtalbotae DuBois. All three species are known only from the type collections. Here, we describe Monomoriumdine Cover & Rabeling, sp. nov., from the Navajo Nation in New Mexico, USA, a new species closely similar to the three known social parasites. Like them, M.dine appears to be a workerless inquiline that exploits a free-living Monomorium host. We also provide keys to the queens of the Nearctic Monomorium inquilines, provide the first images of these species, report new collections for Monomoriumtalbotae DuBois, discuss host-parasite associations, and summarize what is presently known about these mysterious social parasites.