Following a geological study of bottom and core samples of fossil Holocene freshwater thecamoebians involving thousands of specimens, almost all Arcellacea, the authors revise the taxonomy of several confused genera. Arcellacean tests intergrade morphologically between the main phenetic clusters, which has led specialists to treat nearly every rare morphotype as a distinct species or "variety"; the authors attribute this intergradation to the rarity of sexual events and discuss criteria for recognising species in uniparental organisms. Further difficulties arise from the absence of diagnostic soft-part characters in fossils, and from the historical practice of defining species and varieties by the nature of their xenosomes — whereas idiosome shape may be taxonomically valid, xenosome composition generally reflects particle availability rather than genome-based selectivity. They therefore define arcellacean species as broad phenetic clusters linked by intergradation, drawn from large contiguous populations, and reorganise the group into fourteen species in five genera in place of the several dozen recognised by earlier authors. The species are redescribed and their fossil forms illustrated; since the literature indicates these taxa are cosmopolitan and dominate lacustrine faunas generally, the taxonomy is potentially applicable worldwide.