Many aphid (Aphidae) and treehopper (Membracidae) species depend on ants (Formicidae) for their survival, but few leafhopper (Cicadellidae) species have been reported as myrmecophiles. We studied the Mexican species Dalbulus quinquenotatus Delong & Nault in the field and greenhouse. In Jalisco, Mexico, field populations of D. quinquenotatus increased in size when ants were present on their gamagrass (Tripsacum species) hosts. When ants experimentally were excluded, predatory spiders invaded the gamagrass habitat and extinguished D. quinquenotatus populations. Ants also eliminated two nonmyrmecophilous and potential competitor Dalbulus species from the host-plant microhabitat of D. quinquenotatus. Greenhouse populations of D. quinquenotatus killed gamagrass hosts in the absence of predators and attendant ants. Ants (Formica subsericea Say) on gamagrass regulated captive leafhopper populations by removing nymphs and adults, and kept leaf surfaces free from contaminating honeydew. Ants also protected leafhoppers from experimentally introduced nabid predators; in the absence of ants, nabids eliminated leafhopper populations. Diet influenced the response of ants to populations of D. quinquenotatus. When ants were denied food, they preyed upon and extinguished greenhouse populations of D. quinquenotatus, but when supplied with prey (dead eastern yellowjackets), large numbers of ants attended leafhopper populations that grew in size. Few ants attended leafhoppers if they were supplied with insect prey and honey. Our data strongly suggest that D. quinquenotatus is an obligatory myrmecophile.