The ability of silkworm to ingest mulberry leaf, digest, absorb, assimilate and convert it to silk fiber differs from race to race. In the present study two new bivoltine silkworm double hybrids viz, Aditya and Kasturi, and an improved crossbreed namely Bengaluru Belli were used to analyze the leaf to silk and leaf to cocoon conversion efficiency. The dietary indices like cocoon weight, shell weight, leaf – cocoon conversion rate (LCCR), leaf – silk conversion rate (LSCR) were studied. Compared to the respective controls, the new bivoltine double hybrids (Aditya and Kasturi) and new improved crossbreed (Bengaluru Belli) exhibited significantly higher conversion efficiency. The boil – off loss ratio in new hybrids also exhibited significantly low boil-off loss ratio compared to control. Interestingly, the average filament length was also found to be significantly higher in the new bivoltine hybrids and the improved cross breed compared to their respective control. However, the activity of digestive enzyme improved is non-significant between the races studied, also neither between the instars studied. In the present study a significant difference in rearing performance was observed between the races studied even when they were reared under similar environmental conditions. The racial potential and physiological implications of feed conversion efficiency in these silkworm hybrids are discussed with respect to their suitability.