Uttarakhand is popularly known as bowl of bivoltine silk because of its high quality bivoltine cocoons of international grade. Due to its heritage importance the silk is cultivated since long back especially in Doon valley. The climate of valley for producing silk is very congenial. The weather condition during spring and autumn seasons favors the rearing of bivoltine silkworm breeds because their excellent performance. Dehradun lies in the Doon Valley, on the watershed of the Ganga and Yamuna rivers. Land availability, climate and socio-economic conditions of the region favor bivoltine sericulture. Rearing with conventional combinations like NB4D2 x SH6 and its reciprocals are observed to show promising results recording an average yield of above 45 kg per 100 dfls at farmers’ level. Therefore, considering the potential, popularity and consistency in performance of these component breeds over the years, it was decided to continue maintenance and multiplication of NB4D2 and SH6 at P2, Basic Seed Farm, Sheeshambara, Dehradun for downstream multiplication and further exploitation at farmer level. As such rearing performance of NB4D2 and SH6 was reviewed during spring and autumn crops of the year 2020-21, 2021-22 and 2022 - 23. Observations were recorded on various qualitative and quantitative traits viz. fecundity, hatching percentage, larval duration, ERR (by no.), cocoon yield per 100 dfls (by no. and by wt.), single cocoon weight, single shell weight and shell ratio. Results obtained indicate that the both aforementioned races are performing well over the norms in terms of fecundity, ERR, cocoon yield per 100 dfls as well as other characters under review. In NB4D2 fecundity ranged from 520 - 530, 511 - 518.5; ERR from 7914 - 9113, 6317 - 9041 and cocoon yield per 100 dfls from 40383 - 44703, 28241 - 42374 in spring and autumn seasons respectively, while SH6 the same ranged from 503 - 520, 506.5 - 522.5; 6406 - 9950, 6418 - 9160 and 36997 - 46059, 30336 - 44302 respectively for successive three years.