The numerical abundance, biomass, and taxonomic composition of the micro-zooplankton and chlorophyll, phaeophytin, and total seston levels were determined for six depth intervals within the upper 200 m at 12 locations from approximately 10°N to 12°S latitude on longitude 105°W. Data on the hydrography, phytoplankton inorganic nutrients, larger zooplankton standing stocks, and phytoplankton productivity at the various sites was available through the EASTROPAC program. Average micro-zooplankton volume over the euphotic zone showed a threefold range, from 15 mm 3/m 3 at the southerly extreme of our sampling to 47 mm 3/m 3 near the Equator. The relative taxonomic composition of the micro-zooplankton populations within the three size classes of material separated was similar to that described from earlier studies of California Current populations. Highly significant positive correlations were found between micro-zooplankton abundance and chlorophyll levels in the various depth intervals when ‘groups’ of stations, delineated on the basis of similar hydrographic features, or all stations in the study were considered. Standing stock biomass of micro-zooplankton averaged 34% (range, 23–66%) of that of the phytoplankton, both estimated in terms of dry weight over the euphotic zone at the different sites. This was considerably greater than was earlier indicated for stations in the California Current. Calculations, based on assumptions regarding micro-zooplankton feeding rates, suggested that these small animal plankters might be consuming an average of 70% (range, 39–104%)_of the daily phytoplankton organic carbon production at stations along the transect. The same calculation using data from weekly observations taken between April and September at a site five miles off the La Jolla coast indicated an average of only 23% (range, 7–52%) for this relationship. Micro-zooplankton biomass within the euphotic zone at the various locations relative to the larger zooplankton was suggested to be about an average 24%, but this calculation was complicated by the different depths of sampling for the two populations and the methods of determining their volumes.