While several excellent reviews on arachnidism have been published in recent years (Bertin, 1950; Blair, 1934; Bogen, 1926; Greer, 1949; Machiavello, 1949) these have dealt primarily with the clinical problem of spider bite, and have given little information on classification of the poisonous spiders involved. Although differences in toxicity of venoms of various species of Latrodectus have been reported (Keegan, 1950; Shulov, 1948; Vellard, 1936) demonstrations, of common antigenic properties in venoms of L. mactans, L. indistinctus, and L. geometricus by Finlayson and Hollow (1945) indicates need for better understanding of the relationship between members of the genus. In this paper a brief review of the classification of spiders of genus Latrodectus is presented, and criteria for classification are discussed. This paper is based upon a study of the literature and upon examination of specimens from three sources. Specimens of L. hasseltii, L. geometricus, and the several described subspecies of L. mactans were made available by officials of the United States National Museum. Representatives of the same species, as well as L. tredecimguttatus and L. menavodi were obtained from the collections of the American Museum of Natural History through the courtesy of Dr. W. J. Gertsch. In the personal collection of the writer were specimens of L. hasseltii, L. geometricus, and L. mactans. Examples of L. hystrix, L. pallidus, L. indistinctus, L. revivensis, and L. foliatus were not available for study.