Several habitats where this tiny fish is found are legally protected, including the portion of the Rio Yaqui basin in the San Bernardino Wildlife Refuge, Arizona, where the rarer Yaqui topminnow (P. o. sonorensis) subspecies occurs (4). The Gila topminnow was re-introduced to its native rivers from stock populations as early as 1936 for the purposes of mosquito control, and many re-introductions have since occurred for the purposes of conservation of the species, both into man-made and naturally occurring habitats. In particular, large-scale re-introductions began after a Memorandum of Understanding was established in September 1981 between the U.S. Fish and Wildlife Service, the U.S. Forest Service and the Arizona Game and Fish Commission. However, this re-introduction programme had limited success, with the majority of populations disappearing almost immediately, or surviving only for a few short years. Failure was often due to replacement by invading mosquitofish or subsequent habitat destruction or alteration. Most chosen re-introduction sites were too small to resist environmental changes, and were isolated, limiting genetic flow and repopulation by other sub-populations. As the poor results became increasingly apparent in the late 1980s, conservation effort switched its focus to the protection of natural and re-established populations, and re-introductions into better quality areas (9). Additional conservation initiatives have included the propagation of the species for restocking purposes at Dexter National Fish Hatchery and Technology Centre in New Mexico and Arizona State University (9). In an attempt to protect the species and its habitat, road closures, livestock enclosures, recreation management, fish barrier construction, closure of areas to fishing, and habitat restoration have all also been implemented (9). Provided suitable habitat is preserved, protected and restored in this way, and mosquitofish can be effectively controlled, it is hoped that the Gila topminnow’s high reproductive rate will allow it to rapidly repopulate habitats and flourish once more (7).