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y pulchella (Baird and Girard,1854:29) on the basis of two specimens taken in 1851 from -the Mimbres niver north of Deming, New Mexico. Alth©ugh the original description gave the type locality as "Rio Mimbres, tributary of the Gila", which was subsequently changed (Girard, 1859:63) to "Rio Mimbres, tributary of Lake Guzman, Mexico", careful study by William J. Koster of the route of the collector, John H. Clark, showed that the specimens actually came from New Mexico. There is no Rio Mimbres in 'Chihuahua, Mexico, despite Meek's (1904: map opp. xlvii) placement of it there. The Chihuahua chub was next described (Girard, 1856:207) as Tigoma nigrescens from the Rio Casas Grandes basin (Rio Janos, Boca Grande) in Chihuahua, Mexico. Although pulchella is the older name, it cannot be used because Gunther (1868:207) united Gila with Leuciscus and thus brought into secondary homonymy Gila pulchella and Leuc scus pulchellus Storer, 1839 (a synonym of the eastern North American fallfish, Semotilus corporalis). Articles 57 and 59b-c of The International Code of Zoological Nomenclature make unavailable for use any secondary homonym rejected prior to 1960. Jordan and Evermann (1896:234) also rejected Gila pulchella and chose Tigoma nigrescens on the basis of secondary homonymy, although their concept of Leuciscus nigrescens included several currently recognized species of Gila and their description was based chiefly on Gila pandora. The demise of the Chihuahua chub from the United States portion of its range was twice reported prematurely. Koster (1957:57) said it was gone from the Mimbres River, probably because of the introduction of the longfin dace, A osa chrysogaster Girard. Miller (1961) "confirmed" its disappearance on the basis that Carl L. Hubby and family collected in the Mimbres River in 1938, y took no chubs, and were told by a Dr. Royal that in 1884, when the river had more water and many deep holes, he commonly caught 6-inch "Gila trout", the local name for Gila nigrescens. At the time of the Hubby' visit no 'longfin dace were found but Notropis formosus (now extinct in the Mimbres) was taken. In May and June, 1975, 124 years after its last collection there, Gila nigrescens was "rediscovered" in the Mimbres River by Bill Rogers, ahigh-school science teacher and principal under contract for stream-survey work with the New Mexico Department of Game and Fish (Rogers, 1975, unpubl. rept. cited by Hubbard et al., 1978). Two specimens were obtained.. In late March 1979, 10 adult chubs were removed from the river by Michael D. Hatch, employee of the same organization, for propagation at the Dexter National Fish. Hatchery in Chaves County, New Mexico, a federally-supported endangered species facility. The transplanted stock has spawned successfully in a half-acre pond and it is planned to reintroduce the chub into ,the Mimbres after a suitable refuge is made available. The present paper discusses the current population status and habitat requirements of the Chihuahua chub throughout its known range in four disjunct rivers and the endorheic basin of Laguna Bustillos in New Mexico and Mexico (Fig. 1). All Mexican localities are in northern Chihuahua. A survey was made from 4 May to 4 June 1979, financed by the Albuquerque office of the U.S. Fish and Wildlife Service and the U.S. Army Corps of Engineers (Contract Nd. 14-16-0002-79-142). We found the species at 8 of 16 reported localities, but it was abundant at only 3 of these stations -essentially a single station (numbers 5 and 6) in the Rio Casas Grandes and two (numbers 8 and 9) in the Rio Santa Maria (Fig. 1). Abbreviations used below are: ASU (Arizona State i~