Earlier breeding experiments undertaken at Central Sericultural Research and Training Institute, Mysore, India since a decade had resulted in the development of many productive and qualitatively superior bivoltine hybrids. However, the hot climatic conditions of tropics prevailing particularly in summer are not conducive to rear these high yielding bivoltine hybrids. This has necessitated breeding of compatible bivoltine hybrids for year-round rearing. Accordingly, the Japanese hybrid, B2Ol ${ imes}$ BCS12 which was found to be tolerant to high temperature was used as breeding resource material. Following hybridization and selection rearing of silkworms was taken up in SERICATRON (Environmental chamber with precise and automatic control facilities for uniform maintenance of temperature and humidity) at high temperature of $36{ imes}1^{circ}C$ and 85${ imes}$5% RH in fifth instar and the control batches at $25{ imes}1^{circ}C$ and 65{ imes}$5% RH. Directional selection was resorted to the batches reared at 36$pm$1$^{circ}C$ till F$_{5}$ keeping pupation rate as important selection criteria. From $F_{2}% onwards the experiment was modified in such a way as to conduct normal rearing every alternate generation to regain the lost vitality due to continuous exposure to high temperature and high humidity stress. At $F_{2}$ , Oval and dumb-bell cocoons were separated out and designated as CSR18 and CSR19, respectively. By utilizing these lines at $F_{12}$, the hybrid CSR18$ imes$CSR19 was prepared and studied for the thermotolerance by subjecting to stress condition at high temperature of 36$pm$1$^{circ}C$ and 85$pm$5% RH in fifth instar and the control batches at $25{ imes}1^{circ}C$ and 65${ imes}$5% RH. The better performance of CSR18${ imes}$CSR19 (survival > 88%) at $36{ imes}1^{circ}C$ clearly indicates the general superiority of CSR18${ imes}$CSR19 with regard to high temperature tolerance over the productive hybrids and CSR18$ imes$CSR19 can perform well in varied agro-climatic conditions of the tropics with optimum qualitative and quantitative characteristics.s.