Screening of fifteen bivoltine silkworm breeds of Bombyx mori Linn at a temperature of $31{pm}1^{circ}C$ and relative humidity of 85${pm}$5% resulted in the identification of eight thermo-tolerant breeds. The survival and cocoon shell ratio of the tolerant breeds ranged from 72.7 to 78.7% and 20.0 to 20.1% respectively. The tolerant breeds comprised of four oval breeds and four dumb-bell breeds. Eight foundation crosses prepared by crossing the oval and dumb-bell parents among themselves were screened at a temperature of $31{pm}1^{circ}C$ and relative humidity of 85{pm}5%. The performance of the foundation crosses on 11 economic characters were analysed by employing Multipie Trait Evaluation Index method. Four foundation crosses which scored average index value > 50 were selected as breeding parents and breeding initiated for the evolution of thermo-tolerant bivoltine silkworm breeds. The methodology and the results of the foundation crosses reared both at 31{pm}1$^{circ}C$ and at $25{pm}1^{circ}C$ temperatures, are discussed.