Common names: Baja killifish, San Ignacio killifish (E), Sardinita peninsular (S). Conservation status: Threatened, according to SEDESOL (1994) on the basis of its confined distribution and low abundance. Identification: Dorsal and anal fins with 12 rays, eye diameter 4.5 in head, mouth size 4.1 in head, scales in lateral series 34–38 (mean= 36), and breeding males with ctenii of lateral scales strongly developed (Myers 1930); allozymic analysis has identified three diagnostic loci Pgm100, Pt-2102, and Aat-2102 (Camarena-Rosales et al. 1999). Photograph by Gorgonio RuizCampos from Arroyo (Oasis) La Purisima (Carambuche), March 1998, breeding male (total length 82.2 mm). Distribution: Type locality is oasis of San Ignacio, Baja California Sur, Mexico (Vaillant 1894, Myers 1930). Endemic to the oases of the Pacific drainage of Baja California Sur, Mexico, from San Ignacio (Myers 1930, Follett 1960) to Arroyo Las Pocitas (G. Ruiz-Campos unpublished data). The author and collaborators have collected (period of 1991–1998) this species in the following localities: San Ignacio (San Ignacio), La Purisima (Carambuche), Bramonas (Bebelamas, San Luis Gonzaga and Las Cuedas), San Pedro (San Pedro de La Presa, San Basilio, Merecuaco, Pozo del Iritu), and Las Pocitas (El Caracol). Abundance: Although there are no quantitative evaluations of the abundance of F. lima before the introduction of exotic fishes to the oases (e.g., Cyprinus carpio, Poecilia reticulata, Xiphophorus helleri, X. maculatus, and Tilapia cf. zilli), current monitoring (1991–1998) along its historic range has revealed low relative abundance (0–5%) of this native killifish as compared to exotic species, mainly T. cf. zilli and P. reticulata (G. Ruiz-Campos unpublished data). The least decimated population of F. lima was detected in the locality of Las Cuedas (Bramonas basin). Habitat and ecology: The oases that this species inhabits register low salinities (0.3–1.0 ppt), and their bottoms are formed of bedrock with sandy/gravel patches. Aquatic macrophytes are represented by Chara sp. and Potamogeton sp. (Espinosa Perez & Castro-Aguirre 1996). San Ignacio killifish prefer pool habitats with lateral scour (0.3–2.0 m deep) and temperatures ranging between 20 and 28◦C (G. Ruiz-Campos unpublished data). This freshwater killifish is similar to other relatives obtaining food by browsing peryphyton of mainly filamentous green algae and diatoms, and associated ostracods and cladocerans (Alaniz-Garcia 1995). Reproduction: No studies on breeding aspects of this fundulid species have been made; however, observation of males with nuptial coloration and strongly developed ctenii from March to April suggests reproduction during this time (G. Ruiz-Campos unpublished data). Threats: Since this killifish is one of only three known freshwater taxa endemic to Baja California peninsula, its very limited distribution merits considetarion at least as threatened. Recent distribution evaluations (1991–1999) along its historic range and adjacent areas indicate significant reductions of populations in most of localities due to competitive exclusion by exotic cichlid T. cf. zilli (G. Ruiz-Campos unpublished data). Stocking of exotic fishes (mainly Tilapia spp. and Cyprinus carpio) in the oases dates from 1976 as part of a federal program to promote rural aquaculture, with the harmful results mentioned above. Conservation recommendation: Although eradication of Tilapia spp. and other exotic fishes from the oases is unrealistic, because of the size and distance among them; the transplanting of F. lima to pristine oases inside its native range (Hendrickson & Brooks 1991) might be an appropriate strategy to recover this endemic taxon. Remarks: Based on allozymic studies, F. lima is closely related to F. parvipinnis brevis (Camarena-Rosales et al. 1999), a coastal subspecies endemic to Bahia Magdalena, Baja California Sur (Osburn & Nichols 1916). Myers’ (1930) original suggestion that F. lima is a landlocked derivative of F. parvipinnis was supported by Miller (1943) and very recently by Camarena-Rosales et al. (1999).