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Bahía Concepción is one of the largest coastal bodies of water on the peninsular side of the Gulf of California, which is characterized by great fish species diversity. Spatial distributions of fish larvae in Bahí a Concepcioó n during June and November 1997 were analyzed; these months were representative of the extreme hydrographic conditions during an annual cycle in the Gulf. Zooplankton samples (333-mm conical net) and conductivity, temperature, and depth data were obtained at each sampling station. The Bray-Curtis dissimilarity index defined three groups of stations in June (mouth, central, and interior) and two in November (mouth and central-interior), which vary in species composition and dominance. In June, Gerreidae (Eucinostomus gracilis) and Clupeidae (Opisthonema sp.) larvae were the dominant species in the bay mouth; Sciaenidae type 1, Clupeidae (Harengula thrissina), and Pomacentridae (Stegastes rectifraenum) larvae were the dominant species in the central bay; and Gerreidae (E. dowii) larvae in the bay interior. The differentiation of three groups is associated with variations in hydrographic conditions recorded from the mouth to the bay interior, coinciding with a well-defined thermocline throughout the bay as a result of weak winds prevailing in the central Gulf region. In November, Mullidae and Clupeidae (Etrumeus teres) larvae were the dominant taxa in the bay mouth, and Gobiidae (Ilypnus gilberti) and Blenniidae (Hypsoblennius gentilis) larvae dominated in the central and interior bay. The similarity of the larval composition of the central and interior bay is associated with a straight spatial gradient of temperature and salinity and homogeneity in the water column; this condition was caused by strong winds and tides that affect the region in late fall. In addition, the presence of mesopelagic species (e.g., Vinciguerria lucetia) in the bay interior during November indicates a clear influence of the Gulf waters in the bay at that time, possibly as a result of intensive mixing.