Heterosis in hybrid rice was studied in the 1995 wet season under irrigated conditions. Some 22 F1 hybrids developed by using cytoplasmic male sterile lines IR58025A, PMS1A, PMS2A, PMS3A, and PMS10A and 18 restorers were grown along with the standard variety HKR126 in a randomized block design with three replications. Each line was planted in a single row 3 m long. Spacing between rows and plants was kept at 30 15 cm. Observations were recorded for grain yield plant-1, panicles plant-1, grains panicle-1, and 1,000-grain weight.