Leptus clarki sp. nov. is described and separated from the two previously described North American larval Leptus (Acarina: Erythraeidae), and other American species. It is the second larval Leptus recorded as an ant ectoparasite. Larval Erythraeidae rarely parasitize Hymenoptera. Sites of attachment on the harvester ant Pogonomyrmex salinus Olsen are the dorsal to dorsolateral surfaces of the head and thorax. Up to seven mites per ant are recorded