Observations were made in order to learn if the uptake of NH_4 and NO_3 by freshwater plankton can be described by the Michaelis—Menten expression. Uptake of NO_3 and NH_4 by reservoir plankton was estimated at 5 concentrations using (15)N Tracer techniques. A hyperbola results when the uptake velocity (v) of NO3 and NH4 is plotted against concentration (S). The S/v vs S transformation of the Michaelis—Nenten expression was used to estimate K_s. For a mixed population of blue—green algae in Lake Carl Blackwell, Oklahoma, K_s was about 43 mg NO_3 — N (m)(—3), when the initial concentration was 7.53 mg NO_3 — N (m)(—3). In Lake Keystone, where the initial concentration of NO_3 — N was 419.17 mg (m)(—3), enrichment with NO_3 increased v in a similar way, suggesting use of the Michaelis—Nenten model may not be realistic.