The larvae of four ascaridoid nematodes, Toxascaris leonina, Baylisascaris procyonis, Lagochilascaris sprenti, and Hexametra leidyi, were described to allow a specific diagnosis to be made if they are ever found to be causative agents of visceral larva migrans. Larvae of T. leonina, B. procyonis, and L. sprenti were recovered from experimentally infected mice after the larvae had reached their maximum size in these animals, i.e. 32, 14, and 75 days after infection, respectively. H. leidyi larvae used for in toto studies were collected on day 159 postinfection from a rhesus monkey, Macaca mulatta, while H. leidyi larvae to be used for histologic studies were collected from mice when they had reached the same size as those found in the monkey, i.e., at 23 days postinfection. Larvae to be examined in toto were collected by pepsin digestion, fixed in glacial acetic acid or a mixture of equal parts 10% formalin and 0.85% NaCl, and cleared in glycerine. Tissues for histological study were fixed in 10% formalin or Bouin's fixative. All sections were stained with hematoxylin and eosin. The differences in the anatomy between the larvae of the four species when studied in toto, were found to be the length of the body, shape of the anterior end, and shape of the tail. The major differences in the histologic anatomy of these larvae were found to be the diameter in sections, presence of lateral alae, presence of internal cuticular bars, shape and appearance of the excretory columns, and number of intestinal cells present per transverse section. The descriptions of the larvae are compared to and differentiated from other ascaridoid larvae that have been described which may cause visceral larva migrans. Also, features of systematic importance, such as the shape of the esophageal gland cell nuclei and shape of the excretory system are discussed as to their apparent importance for separating the two subfamilies, Ascaridinae and Augusticaecinae, sensu Hartwich, 1974. It is concluded that until such features are examined on more species of related ascaridoids that the classification of the Ascarididae sensu Sprent, 1983, should be used