The gastrointestinal tracts of 167 of 489 (34%) Yarrow's spiny lizards (Sceloporus jarrovii jarrovii) collected in Pima County, Arizona (USA) from October 1967 through January 1970 were infected with Physaloptera retusa. Of the infected lizards, 88 (18%) had only larvae, 45 (9%) had larvae and adults and 32 (7%) had only adult nematodes. The ratio of larval nematodes to adults was approximately 8:1. Monthly prevalences varied from 71% in April 1969 to 10% in July 1969; seasonal patterns of infection were not observed.