Onion (Allium cepa L.) ranks second in importance after potato among the vegetables in India. Optimum irrigation scheduling for onion and its response to irrigation based on the ratio of irrigation water (IW) and cumulative pan evaporation (CPE) at 1.2, 0.9, and 0.6 were studied in a field experiment during 1994-95 and 1995-96. The site had a sandy loam (Typic Fluvaquent) soil, located in the humid tropical climate at the Central Research Farm, Gayeshpur (23° N, 89° E, and 9.75 m above mean sea level) in West Bengal of eastern India. The yield of onion, daily and seasonal actual evapotranspiration (ETa) from water balance equation, crop coefficients (Kc), soil water stress coefficients (Ks), water use efficiency (WUE), irrigation water use efficiency (IWUE), and water application efficiency (Ae) were determined. Maximum profile water depletion and ETa of onion occurred in the wetter moisture regime (1.2 IW:CPE). The average daily ETa varied from <1 mm day-1 in the early growing period to a maximum value of 2.92 mm day-1 at the mid-season under 1.2 IW:CPE. The seasonal ETa of onion with IW:CPE of 1.2 was 254.82 mm and 243.49 mm during 1994-95 and 1995-96, respectively. The bulb yield increased significantly (P < 0.05) with increasing levels of irrigation. The IW:CPE ratio of 1.2 produced 34 and 79% more yield and 27 and 62% improved WUE than 0.9 and 0.6 IW:CPE, respectively. Irrigation regime of 0.6 1W:CPE indicated lower IWUE and higher Ae. Precise information of KC, which is required for regional scale irrigation planning is lacking in developing countries. The estimated values of Kc for onion at initial, crop development, midseason, and maturity were 0.52, 0.85, 1.04, and 0.87, respectively, which were identical to those suggested by the Food and Agriculture Organization, indicating the need for generating these values at the local and (or) regional level. The Ks values of 0.6 IW:CPE decreased steeply and experienced more stress condition than 0.9 IW:CPE. The observed differences in water stress would be expected to influence yields. Furthermore, second order polynomials were presented to predict the Kc values from days after transplanting and growing degree-day.