The two known social parasites of fungus-growing ants (tribe Attini) occupy an advanced grade of social parasitism characterized by absence of a worker caste and highly derived morphology and behavior. In contrast, the Panamanian Acromyrmex insinuator new species, described here, appears to occupy an early grade of social parasitism in which males, females, and minor workers are nearly indistinguishable from those of the host species. Based on allozyme and morphological evidence, the host, A. octospinosus ssp. echinatior, is clearly different and reproductively isolated from the sympatric A. octospinosus ssp. octospinosus, and is therefore elevated to species status.