The California grunion, Leuresthes tenuis (Ayres 1860), is a common atherinid fish that schools near shore, usually near the surface, along the open coast and bays from San Francisco, California (USA) to Bahía Magdalena, Baja California Sur (Mexico) (Moffatt and Thomson, 1975). It has a recreational value due to its peculiar behavior of spawning on sandy beaches (Thompson, 1919). Grunion spawns from February through August, peaking in April-May, with a semilunar periodicity. Eggs incubate in the sand for about 2 wks and hatch when the next series of high spring tides stir the sand (Thompson, 1919; Walker, 1949). On hatching, larvae are large (6.7-8.9 mm), well developed, and very active (David, 1939; Hubbs, 1965; May, 1971). It has little commercial value, but is significant as prey for birds and economically important fishes in the region (Schultze, 1983). Grunion eggs and larvae have been used commonly in experiments to determine factors affecting embryonic and larval growth and survival (Ehrlich and Farris, 1971; May, 1971; Reynolds et al., 1976, 1977; Ehrlich and Muszynski, 1982; Winkler et al., 1982; Rosales-Casián et al., 1990; Vásquez-Yeomans et al., 1990; Vásquez-Yeomans and Sosa-Cordero, 1992; Rosales-Casián, 1994). Larval development of L. tenuis has been described by Wang (1981) and Watson (1996) but its developmental osteology has not been studied. Descriptions of osteological structures and their development are valuable in phylogenetic studies (Dunn, 1983). Hence, the purpose of this paper is to describe the osteological development of the caudal complex of larval L. tenuis, emphasizing differences with other atherinid species. Inglés