A survey of zooplankton in the upper 300 m of the central California Current, from the coastal shelf to 128W and between 36.5N and 39.5N, was conducted in early June 1993 using an Optical Plankton Counter (OPC) as part of an interdisciplinary study of mesoscale ocean circulation and biological dynamics. The OPC was part of a multi-instrument package towed on an undulating vehicle (SeaSoar). Estimates of normalized size spectra and absolute abundance from OPC data compared favorably with measurements based on Bongo net catches. After processing, the standardized OPC data set provides a resolution of ≈7 km in the horizontal and 10 m in the vertical for 60 size categories of zooplankton ranging in estimated body weight from 3 μg C to 3000 μg C. Results reveal rich mesoscale variability in zooplankton distributions at scales of 30–60 km, with regions of enhanced biomass and abundance coinciding with the location of mesoscale eddies, both cyclonic and anticyclonic. The central jet of the California Current departed from the coastline near Cape Mendocino (39.5N), separating a region of generally greater zooplankton biomass near the coast from one of lower biomass to the west. The jet usually contained lower biomass than adjacent eddies. Total zooplankton biomass ranged from <2 g C m−2 to >20 g C m−2. The two mesoscale features containing the highest biomass, one 200 nm offshore of Cape Mendocino and another 100 nm west of Monterey Bay, were dominated by euphausiids.