A breeding programme was initiated by utilizing a robust bivoltine breed CSR$_{18}$ and a polyvoltine breed Cambodge with the main objective of developing robust polyvoltine silkworm breeds/hybrids. At F$_1$ and F$_2$, parthenogenetic development was induced following warm water treatment of eggs at 46$^{circ}C$ for 18 min followed by two backcrosses with Bl$_{67}$, an evolved polyvoltine breed. The newly developed breed was subjected for hybrid study using eight hybrid combinations in the laboratory at F$_{8}$ generation. F$_1$ hybrids between newly developed breed Bl$_{67}$ (Pg) and promising bivoltine breeds exhibited their superiority by expressing significant hybrid vigour for several economic characters like cocoon yield/10,000 larvae, cocoon weight, cocoon shell weight, cocoon shell ratio and denier. Study on cocoon shape variability revealed that cocoons of all the F$_1$ hybrids except BL$_{67}$ (Pg)${ imes}$NB$_4$D$_2$ were found comparatively uniform in shape.pe.