Field experiments were conducted during the 1993 and 1994 dry seasons (June-September) to compare the effect of inoculation of Azolla hybrids and fertilizer N on rice yield. The experiment was laid out in a randomized block design with three replications. The test soil was silty clay with a pH of 6.4, EC 0.45 dS m-1, organic C 0.57%, 30.20 mol kg-1 CEC, 320 kg available N ha-1 , 24.9 kg P ha-1, and 273.9 kg K ha-1. ADT36 rice seedlings were transplanted in 1.5-m2 plots with 15- × 10-cm spacing. Superphosphate and potash as muriate of potash at 50 kg ha-1 were applied as a basal dose. Nitrogen at 75 kg ha-1 was applied as urea super granules (USG) alone and with Azolla. USG was applied at 10 cm deep at 7 d after transplanting in between four hills of alternate rows, and prilled urea (PU) alone was applied in four split doses (40, 20, 20, and 20%) at basal, tillering, panicle initiation, and flowering stages for comparing with USG. Azolla hybrids AH-C1 ( A. microphylla 4018 /A. microphylla 4028 (V3)), AH-C2 ( A. microphylla 4018 /A. microphylla 4028 (V4)), and AH-C3 (strain of A. microphylla from China), and wild type A. microphylla (from Galapagos Islands) were largest grain and straw yield for ADT36 in both seasons compared with all other Azolla cultures.