The external morphology of the adult, egg, larva, and pupa of Agathidium oniscoides Beauvois is described. The natural history of the genus is reviewed, including associations with Myxomycetes and fungi and techniques for field collection and laboratory study. The phylogenetic species concept is discussed and applied in this study. A key and brief diagnoses are provided for the genera of Agathidiini (Coleoptera: Leiodidae: Leiodinae) of North and Central America. Volvoxis Kugelann is regarded as a nomen oblitum, and its junior synonym Agathidium Panzer is regarded as the valid name (nomen protectum) for this taxon by invoking article 23.9 of the International Code of Zoological Nomenclature. A key to species groups of Agathidium Panzer is presented. Species are revised for the A. sexstriatum, A. brevisternum, and A. revolvens species groups, including keys, diagnoses, descriptions, and figures of important morphological features. Lectotypes are designated for A. bistriatum Horn and A. sexstriatum Horn. The following new species are described: A. angustoperculum, n.sp., A. dioperculum, n.sp., A. dubitanoides, n.sp., A. falcatoperculum, n.sp., and A. rhinocerellum, n.sp.