Extracellular hydrolysis of 4-methylumbelliferyl beta-N-acetylglucosaminide was measured in the oligomesotrophic Piburger See and the eutrophic Rimov reservoir during spring and summer phytoplankton blooms. respectively. Total enzymatic activity (TEA) ranged between 0.2 and 19.1 nmol l(-1) h(-1) in the reservoir and between 0.8 and 12.4 nmol l(-1) h(-1) in the lake. High-affinity (K-m < 1 mu mol l(-1)) and low-affinity (K-m > 100 mu mol l(-1)) enzymes were kinetically identifiable in most samples from both localities. The low-affinity enzyme activity (LEA) usually accounted for >60% (mean: 80%) of TEA. LEA and diatom biomass significantly correlated over time in the reservoir epilimnion (r(s) = 0.578) and in the lake metalimnion (r(s) = 0.862). As diatoms possess chitin and take up its monomer, N-acetylglucosamine, two explanations of the observed relationships are suggested: extracellular beta-N-acetylglucosaminidase activity partly originates either from ectoenzymes of chitinolytic bacteria attached to diatom cells or from ectoenzymes of diatoms, enabling them to take up N-acetylglucosamine from ambient amino sugars instead of synthesizing it de novo. A significant positive correlation of LEA with crustacean abundance was found in the lake epilimnion (r(s) = 0.850), apparently reflecting the growing spring populations of frequently moulting juvenile crustaceans. A possible contribution of chitinolytic bacteria, accompanying the crustacean populations, to LEA is discussed.