ABSTRACT Larval fish composition and abundance were analyzed during February-March (winter) and November-December (autumn) of 2005 to investigate which environmental conditions and mesoscale oceanographic processes affect the distribution of larval fish assemblages in the Gulf of California, and if these variables and processes are useful to delimit specific regions of ichthyofaunal distribution. The overall pattern was mostly a latitudinal gradient of the abundance of two faunistic complexes of larval fish assemblages (temperate northern and tropical southern complexes) strongly associated with the spatial distribution of sea surface temperature. The integration of these results, with previous investigations in the region, lets us confirm that the northern and southern larval fish assemblages extend and contract their distribution ranges according to the latitudinal displacement of the 21°C sea surface isotherm. We identified this to be a valuable criterion to establish the southern distribution limit of the temperate larval fish assemblage, and the 18°C as the northern limit of the tropical larval fish assemblage. However, this general pattern is maintained only in absence of mesoscale oceanographic features (upwelling and eddies) that increase larval drift from the coast to the central region of the Gulf, particularly during the November-December period when longitudinal gradients were stronger than in February-March.