Four species are known in the atractid genus Cyrtosomum Gedoelst, 1919: C. scelopori Gedoelst, 1919; C. penneri Gambino, 1957; C. readi Gambino, 1958; and C. heynemani Gambino, 1958. Members of this genus of pinworms are found exclusively in the lizard family Iguanidae. They are apparently further restricted within that family to the North and Central American forms, judging from collecting records available (Pereira, 1935; Karve, 1938; Lent and Freitas, 1948). Of the 765 specimens of iguanids examined for Cyrtosomum spp., approximately 17 percent or 132, were infected. These infections were distributed as follows: C. penneri in 48 lizards (Table I), C. heynemani in 36 (Tables II and V), C. readi in 25 (Tables III and V), C. scelopori in 18 (Table IV). Dual infections of C. heynemani and C.' readi (Table V) were found in 5 host specimens. All but one of the North and Central American genera of iguanids commonly recognized were represented, a total of 46 species and subspecies having been examined. Among these hosts, C. penneri was found in 7 species, C. heynemani and C. readi in 5, and C. scelopori in 6. A single species was found with the dual infection. Table VI lists the 17 species which were found to be uninfected. With the few exceptions shown in Table V, each species of parasite is specific for several closely related host species. The restriction of each of these nematodes to groups which are closely related taxonomically attests to the high degree of host-parasite specificity exhibited by the genus Cyrtosomum. The low incidence of dual and cross infections, despite considerable distributional and ecological overlap of the hosts, indicates a specificity that appears to be physiological rather than ecological. This is best illustrated by the material collected in the southern deserts and coastal mountains of California. Examination of over 300 lizards from this limited geographic region revealed neither dual nor cross infections, yet three of the four species of Cyrtosomum appear to have foci of infection in this area.