We studied the relationship between reproductive phase and heterosis and looked for traits affecting heterosis. In the 1994 wet season at the International Hybrid Rice Observational Nursery (IRHON), 24 hybrids and their respective restorers (one plant each) were dissected to identify the initiation of reproductive phase marked by the formation of a hairy structure. Similarly in the 1995 wet season, three to five plants each from 20 hybrids and their 20 restorers were dissected. At maturity grain yield m -2 and observations on several yield components in five selected plants were recorded as well as days to 50% flowering for each hybrid and restorer (Table 1). Reproductive phase initiation occurred early in hybrids. Restorer yield in parents was higher than maintainer yield in all hybrid combinations. We found a definite, positive relation between better parent (restorer) heterosis and reproductive phase duration. (Table 1). In the H>R group, reproductive phase duration of hybrids was extended with no change in growth duration (days to 50% flowering). In group H<R, days to 50% flowering of hybrids was early compared with restorers but was unaccompanied by a change in reproductive phase duration.