Concerted efforts have been made to cross different silkworm varieties to produce the hybrids with an objective to improve the productivity traits. Systematic and planned hybridization together with improved farming in rearing practices in sericulture has helped a great deal to increase the productivity by several folds in sericultural countries including tropical India. In fact, the ultimate results in silkworm breeding are judged by the superiority of commercial characters of the parental strains that appear in the F1 hybrids in the form heterosis or the extra vigor which form paramount criteria in the evaluation of the suitability of hybrids for commercial exploitation. Hence, in the present investigation, an attempt has been made to select superior bivoltine hybrids utilizing the four newly evolved bivoltine races of the authorsÂ viz.MG405,MG406,MG408 andMG414 and crossing them with four conventional bivoltine races viz. KA, NB4D2, NB7 and NB18, to evaluate the rearing performance of different hybrid combinations for heterosis, over dominance and also determine their extent improvement over the control hybrids. The manifestation of heterosis was observed to be varying with the different economic traits and the hybrids. The extent of heterosis was high for productivity traits than the viability traits. Higher level of heterosis and over dominance was noticed for the traits, cocoon yield by weight, single cocoon weight, shell weight and filament length. Majority of the new hybrid combinations manifested better heterosis and over dominance values than the two control hybrid combinations, NB18 x NB7 and KA x NB4D2 for most of the characters studied.