Zooplankton populations in the St. Johns River Estuary ranged from 3,000/m3 to 20,000/m3 during monthly collections from September, 1973 to August, 1974. Acartia tonsa Dana was the dominant organism throughout the year, except for spring blooms of Balanus sp. nauplii in the lower estuary and summer occurrences of Eubosmin tubins in the upper estuary. With these two exceptions, species composition, and community structure were similar throughout the estuary, year round. A patchy distribution of the zooplankton caused high replicate sample variation which made it difficult to detect small population variations. Salinity appeared to have the greatest effect on the distribution and abundance of zooplankton within the estuary. Population numbers were sufficiently low to suggest that zooplankton were not the major component of secondary production.