The extent of heterosis was examined for six agronomic and two chemical characters in a complete diallel cross experiment involving a set of ten self-pollinated varieties of sesame (Sesamum indicum L.) of diverse ecogeographic origin. The magnitude of general and specific combining ability variances and differences in reciprocal F 1 hybrids were also investigated.Heterosis was, on average, highest for seed yield (33%), followed by number of capsules per plant (16%). Mean percent heterosis was small for earliness, plant height and number of primary branches, while it was insignificant for number of secondary branches and percentage of oil. Significant negative heterosis was observed for percentage of protein.Percent heterosis was generally higher in Indian x exotic crosses than in Indian x Indian and exotic x exotic crosses. The exotic lines appeared to be useful in Indian sesame breeding programmes, particularly in the improvement of earliness, number of secondary branches, seed yield and protein content.Additive as well as non-additive gene action seemed to govern the expression of the various characters studied. General combining ability variances were predominant for days to flowering, plant height, number of primary branches and number of secondary branches, while specific combining ability variances were in moderate to high proportions for seed yield, percentage of oil and percentage of protein. T.M.V.-2 and S. I. 770 were the best general combiners for many of the characters while S. I. 1783 and Sel-R were found to be the best for earliness and oil content respectively. Significant variances due to reciprocal effects were also detected for some of the characters.Isolation of pure lines was preferred to production of hybrids in sesame, and crossing selected sibs in the early segregating generations followed by inbreeding seemed to be a desirable breeding procedure.