A taxonomic key is presented for the identification of mature larvae of cleptoparasitic bees to family, subfamily, tribe, and genus/subgenus. A total of 39 genera/subgenera are treated. All taxonomically important characters are illustrated. A table cites references to mature larvae described since the appearance of “A catalog and review of immature Apoidea (Hymenoptera)” (R. J. McGinley, 1989, Smithson. Contrib. Zool. 494: 24 pp.). Other tables list the known or strongly suspected cleptoparasite/host associations at the tribal, subfamilial, and familial levels. A final table lists the genera/subgenera of cleptoparasitic bees whose mature larvae have yet to be found and studied.