Data from an experiment involving six polyvoltine and three bivoltine silkworm breeds have been analysed to know the level of hybrid rigour and cocoon shape variation in Fl hybrids among silkworm breeds with lows medium and high cocoon weight. Results indicated significantly positive heterosis for several quantitative characters with maximum total heterosis over mid parent value in hybrids between low and medium followed by low and low, medium and medium, low and high and high and low cocoon-weighed breeds, respectively. Study on cocoon shape measurement revelled that among Fl hybrids, cocoons of the three combinations such as GNP ${ imes}$ CSR2, GNP ${ imes}$ J2 and PM ${ imes}$ NB4 D2 were comparatively uniform in their cocoon shape. Importance of this study to know the level of heterosis and cocoon shape variability in different silkworm crosses hale been discussed.