Abstract: Expression and identification of putative Cry proteins of the entomopathogenic bacterium Bacillus thuringiensis israelensis (Bti) have been documented during 24, 48, 72 and 96 h of bacterial culture both at 30 and 37°C. Putative Cry11A and CytA, Cry4B and Cry4A have been recorded to be early, mid and late expressing proteins, respectively. No difference on the level of expression of Cry proteins has been observed between 30 and 37°C. The mosquitocidal proteins were found to be very thermo labile and crude proteins are effective against Aedes aegypti larvae than partial purification. These studies showed that both Cry4B and Cry4A are very unstable even at 40°C and therefore synergistic interactions among the Cry proteins with that of CytA which is required for high toxicity is probably hampered. These factors probably responsible for the total loss of mosquitocidal efficacy of the heat-treated partially purified proteins of Bti. Heat-treated proteins were found to loose the mosquitocidal efficacy against the larvae of Aedes aegypti mosquito.