A study was made on the effects of irrigation and fertilization on the yield, number, and size of potato tubers. Irrigation was based on the mean soil moisture tension of 0.3, 0.6, 1.0, 3.0 and 10.0 bars. Fertilizer in 40 kg/ha of N, and K(2)0 and 80 kg/ha of P2Os, being the dosis 1, 40-80-40 kg/ha, of N, P(2)0(5), and K2O, respectively. Better yields were achieved at soil moisture tensions lower than 1.0 bar and fertilizer dosis 3,4 and 5. There was no significant difference in the number of tubers among the treatment. A increase in the percentage of large tubers was observed in the treatments with soil moisture tension of 0.3, 0.6, and 1.0 bar and fertilizer dosis 4 and 5. On the other hand, a high percentage of small tubers were obtained at higher tensions (over 1.0 bar) and fertilizer dosis 1 and 2.