The time and space distribution of zooplankton biomass recorded during a year cycle (December 1995-1996) off the Pacific coast of central México is analyzed. Samples were obtained by surface (42-86 m) oblique hauls at 12 sampling sites using a Bongo net. The overall average displacement volume biomass of zooplankton during the surveyed period was 1138 cm3 / 1000 m3 . Principal component analysis indicated that highest biomass concentrations occurred at coastal stations. The months with highest biomass values were those in which the lowest sea surface temperature values occurred (January-May). This was the same period in which the California Current was strongest and clearly influenced the hydrological conditions of the surveyed area. In these months, advective processes are active along the outer shelf, favoring upwelling of colder, relatively nutrient-richer waters that promote an overall local increase of zooplankton activity and populations. The high variability of biomass values is indicative of episodic, localized processes that enhance productivity in the area.