At a neap tide, an attempt was made to follow the quantitative and qualitative nycthemeral variations in a planktonic population under the influence of the tide. Simultaneous study was made of changes in the physical medium and the biological material present. Physical measurements at 4 depths, of salinity, density, and speed and direction of current, were made every hr and eventually correlated with plankton samples made at 2 hourly intervals. Physical results are shown in graphs and tables and an analysis given of the zooplankton studied. Examination of the biovol obtained by sedimentation revealed a greater density of organisms at night, copepods forming the principal component of the plankton. Further studies were made of the copepods, various groups being recognized according to the relative effect on numbers of tide and time of day. Population fluctuations were concluded to be related to day-night light intensity alternation, influence of tides, and secondarily, influence of currents.