As a first approach to the study of the dynamics of amino sugars in freshwater environments, we measured microbial uptake of N-acetylglucosamine and hydrolysis of 4-MUF-N-acetyl-beta-glucosaminide by extracellular enzymes in fishponds, reservoirs, backwaters, and lakes. Amino sugars were present in all localities and comprised up to several percent of both dissolved and particulate organic matter. Processes involving N-acetylglucosamine (extracellular enzymes, microbial uptake) were also detected in all localities; their potential rates accounted for approximately 10-100% of the rates of analogical processes involving glucose and were correlated with dissolved organic C and with general microbial activity. Two types of 4-MUF-beta-N-acetylglucosaminidases (high and low affinity) were distinguished kinetically. The activity of the high-affinity enzyme was tightly correlated with variables coupled to bacterial activity. In fishponds, uptake of N-acetylglucosamine by diatoms and cyanobacteria was detected via autoradiography.