The new species is from the digestive tract of the iguanid lizard, Cyclura carinata. A key to the valid species of Cyrtosomum is presented. Data showing overlap in measurements and ratios of Cyrtosomum species from the western United States are given, and C. readi and C. heynemani are synonymized with C. penneri. Gedoelst (1919) erected the genus Cyrtosomum for an atractid nematode parasitizing the colon of certain New World iguanid lizards, and described the first species, C. scelopori. Four additional species have been described: C. penneri Gambino, 1957; C. readi Gambino, 1958; C. heynemani Gambino, 1958; and C. longicaudatum Brenes and Bravo-Hollis, 1960. This paper describes a new species from the digestive tract of an iguanid lizard, Cyclura carinata, and presents a key to the genus Cyrtosomum. "Mega" is a Greek word meaning "big," and large size distinguishes C. mega from all other Cyrtosomum species. MATERIALS AND METHODS