The properties of three amidohydrolases, i.e., urease (I) EC 3.5.1.5, L-asparaginase (II) EC 3.5.1.1, and L-glutaminase (III) EC 3.5.1.2, were studied in sediment samples taken from a shallow eutrophic freshwater lake.Sediment samples were air dried (ADS) and stored for at least 3 months before being enzymically characterized. The pH optimum of I, II, and III were pH 7.0, 8.4, and 6.5-7.0, respectively, while III in soluble extracts from ADS was most active between pH 8.0 and 9.0. The temperature response of the three enzymes in ADS gave Ea values of 38.9, 41.6, and 35.9 kJmol(-1) for I, II, and III, respectively. Km and Vmax values for ADS I, II, and III were 1.2 mM and 1.9μmol NH3 g(-1)h(-1); 0.8 mM and 4.1μmol NH3 g(-1)h(-1); and 1.25 mM and 17.4μmol NH3 g(-1)h(-1). Km values for all three enzymes in ADS extracts were at least an order of magnitude greater than those of the ADS. The susceptability of each enzyme to proteolysis was followed in ADS and fresh wet sediment and compared with that of III in an ADS extract. All sediment enzymes were found to be more resistant than the commercial preparation of bacterial L-glutaminase subjected to the same treatment. These results suggested that I, II, and III all exist to some extent as colloid-immobilized enzyme fractions in freshwater sediments and are analogous to the stable enzyme fractions in soils.