The growth characteristics of Spirulina platensis (K-2 strain), a freshwater isolate from Lake Kojima, was determined for nitrogen utilization. The alga can utilize nitrate, nitrite, ammonia, urea, casamino acids, and some amino acids as nitrogen sources with best growth at urea and asparagine. The stimulating effect of some organic extracts and nucleic acids was observed. Values on the different growth parameters for nitrate, ammonia, urea, and asparagine were the following: half-saturation constant (K_s) of 1.2, 0.25, 0.15 and 0.10mg N/l, respectively; maximum growth rate (μ_max) of 0.48, 0.67, 1.2 and 0.7/d, respectively; and nitrogen level for saturated growth yield of 11, 5, 10 and 8mg N/l, respectively. Results indicated that the metabolic efficiency of the alga is best on urea and is worst on nitrate. The alga was found to have specific growth parameters for each nitrogen source independent of its preconditioning in different nitrogen sources. Enzyme systems for ammonia and urea assimilation seemed to be constitutive but those for aspar-agine assimilation may be induced by depletion of nitrogen or incubation in the sole nitrogen source of asparagine.