Short-term variability in the zooplankton of a shallow estuary with minimal tidal fluctuations in the northwestern Gulf of Mexico was examined at one station on April 13 through 15, 1976. Densities were estimated from three replicate oblique tows taken every four hours over the 44-hour study period. Counting and subsampling error was not significant compared with replicate tow variability, and replicate tow variability was small compared with the variability among sampling times. Overall densities were large during the night compared to the day and the greatest variability in replicate tows occurred during both sunrise sampling times.