The largest difference in an optimum temperature of growth condition, i.e. 27°C for Azolla japonica and 31°C for A. pinnata var. imbricata, was found. The effects of mineral nutrients are combined nitrogen in the medium on the growth and N2 fixation of A. japonica were studied. The optimum concentrations of K, Mg, Ca, P, and S in the medium for growth were about 0.5, 0.1-0.5, 0.5, 0.1 and ... [Show full abstract] 0.05-0.1 m M and those for N2-fixing capacity were about 0.05-0.1, 0.05-0.1, 1.0, 0.5 and 0.05 m M, respectively. Ammonium in the medium inhibited strongly both growth and N2 fixation. The total nitrogen budgets, as a consequence of the balance between nitrogen inputs from atmosphere and medium, was positive at concentrations of ammonium below 10 m M, whereas it was negative above this concentration. Nitrate was less effective than ammonium. Urea addition into the medium resulted in a substantial increase in growth rate and nitrogen content although N2 fixation decreased.