Common Names: Cuatrociengas pupfish, cachoritto de Cuatrociengas. Conservation Status: Listed as Threatened (SEMARNAT 2002). Current IUCN Red List status: Lower Risk/least concern. Identification: Morphologically streamlined. Mature males are chalky blue, have yellow eyes, and a thin, terminal black band on the caudal fin. Females and juveniles have two dark brown lateral lines, with a light blue to pale brown base color. Juveniles and adults have no ocellus on dorsal or anal fin. Mature males range from 30.9 to 51.4 mm SL, mature females from 29.5 to 41.3 mm SL (Miller 1968). Image: modification of mature male holotype in Miller (1968). Distribution: Endemic to the Cuatrociengas basin, Coahuila, Mexico. Abundance: Abundant, with generally large and stable populations throughout its range. Habitat and ecology: Inhabits stable, physicochemically benign thermal springs, outflows, and rivers (Miller 1968; Minckley 1969). Young are omnivorous, but diet is largely plant/ sediment based after 24–29 mm SL (Arnold 1972). Co-occurs with many fish species (Miller 1968) and is stenoplastic (Carson et al. 2008). Often sympatric with C. atrorus in intermediate environments, where natural hybridization is extensive. Ancient hybridization led to complete replacement of C. bifasciatus mitochondrial genome by that of C. atrorus, but nuclear gene introgression from C. atrorus appears minor to absent (Carson and Dowling 2006). Reproduction: Mature males breed and feed in leks in shallow water. Mature females school and feed in deeper water, entering leks to spawn. Breeding occurs year-round, though is reduced in winter (Arnold 1972). Threats: Groundwater extraction is primary. Expanded eco-tourism and introduction of non-native species are also of concern. Conservation action: In 1994 the Cuatrociengas basin was declared a Natural Protected Area by the Mexican government. Conservation efforts since have been weakly-effective (Figueroa and Sanchez-Cordero 2008). Conservation recommendations: Elimination of groundwater extraction is urgently needed. Effective conservation strategies need to be devised and implemented. Environ Biol Fish (2009) 86:445–446 DOI 10.1007/s10641-009-9546-8