A highly significant dependence was found between the number of rotifers and the abundance of euglenoids and green algae. The numbers of copepods chiefly depended on the numbers of green algae and Chrysophyceae and to a very small degree of diatoms and blue-green algae. The total number of zooplankton depended on that of green algae, euglenoids, and desmids. No significant relation was found between Cladocera and 8 groups of the phytoplankton. The prevalence of small algal species was correlated with a greater content of phosphorus. In the case of euglenoids, green algae, desmids, and Chrysophyceae significant negative dependences upon N-N03 were observed. An increase in the number of desmids was associated with low concentrations of silicon in the environment.