Bacillus thuringiensis var. israelensis (Bi.) containing alginate microcapsules were prepared in order to maintain durable formulations which could resist several effects causing reduced efficiency during applications. Bi. spores were harvested through NYSM agar plates and encapsulated in Ca-alginate (0.5-20%w/v) gels without any significant loss of sporal or larvicidal activity. The effect of acidic pH on the larvicidal toxin was tested using Culex sp. larvae in the laboratory. The alginate microcapsules pretreated with saturated KH2PO4 solution gave larvicidal activity after 24-48h, by bioassay. Suspension and encapsulated forms of the pathogenic bacterium were exposed to pH variations (3.0-10.0), UV light and high temperature (50 degrees C). Durability to Pb++, Cu++ Fe++ compounds and phenol was also examined. As the alginate content increased, stability of Bi. drastically increased against the tested effects, but to obtain useful releasing microcapsules, 1.0-1.5% w/v alginate concentrations were found to be optimum.