Host–parasite association among 58 flea species parasitizing 40 mammal species in the Great Basin Desert of the western United States was investigated. Increased flea species richness was correlated with larger geographic ranges and stable locomotion of hosts. Hosts from habitats of moderately low productivity (sage and grass) and of Peromyscus maniculatus size, 10–33 g, had the highest flea species richness. Larger hosts had fewer flea species, but fleas were more prevalent. Increased host species richness correlated with flea species eye size. Mammals clustered into 3 major and 1 minor ecological groups, and fleas clustered into 2 major groups among rodents, and 6 minor groups, forming 12 host–parasite biocenoses. Factors producing biocenoses were shared burrows of mice and rats; food chains of hares, rabbits, squirrels, and their predators; keystone mammals: Lagurus curtatus, Neotoma lepida, Ochotona princeps, and Spermophilus townsendii; keystone fleas: Megabothris abantis, and Meringis hubbardi; or host isolation, Neotoma cinerea with Oropsylla montana, Sorex vagrans with Corrodopsylla curvata, and Tadarida brasiliensis with Sternopsylla distincta. Although host relatedness accounted for flea prevalence, host sociality explained the presence or absence of mammal–flea relationships.