Size fractionated (<0.23, 0.23-2.5, 2.5-100 mum) apparent alpha-glucosidase activity (alphaGlcA), beta-glucosidase activity (betaGlcA), and beta-N-acetylglucosaminidase activity (betaGlcNAcA) were measured using MUF-substrates in the eutrophic Rimov reservoir during one season. The total alphaGlcA (0.7-5.0 nmol.l-1.h-1) correlated with chlorophyll-a, bacterial and cladoceran biomasses, and abundance of heterotrophic nanoflagellates. The total betaGlcA (2.0-7.9 nmol.l-1.h-1) showed a relationship to cladoceran biomass. The particle-bound betaGlcA correlated with bacterial and cladoceran biomasses and chlorophyll-a. The total betaGlcNAcA (1.3-7.4 nmol.l-1-h-1) correlated with copepod biomass. Ecological significance of betaGlcNAcA was supported by N-acetyl-C-14-glucosamine incorporation (V(max): 0.42-1.98 nmol.l-1.h-1), which indicates the ability of microorganisms to utilize the liberated N-acetylglucosamine. Relationships found for the above enzyme activities to zooplankton might be explained by subsequent bacterial hydrolysis of nondigested algal or chitinaceous remains egested by cladocerans and copepods.