The natural history and mating behavior of Pseudodyscrasis scutellaris (Wiedemann) (Diptera: Ulidiidae), a fly associated with Agave atrovirens Karw. ex Salm-Dyck (Agavaceae) in the Mexican Altiplano is described based on a 1-yr monthly survey of adult, pupal, and larval stages and on 108 h of field observations. P. scutellaris overwintered in the egg stage, resumed larval development in the spring, pupated in the soil under agaves, and adults emerged during the summer, Courtship behavior and male agonistic encounters consisted of sequences of short distance visual displays, Females oviposited in feeding scars produced by lepidopteran larvae during previous seasons, Larvae developed in aggregations of >100 individuals the following spring. Adults fed on various carbohydrate and protein sources, frequently interacted with other species of Diptera and ants, and were preyed upon by spiders and wasps. We discuss our findings in light of the evolution of behavior within the superfamily Tephritoidea and highlight the importance of further studies on the community of arthropods found on agaves.