Collapsible soils are prone to considerable volume change as a result of wetting. The presence of collapsible soils under the footing of industrial or residential buildings can bring about irrecoverable and considerable damage to the supporting structures due to settlement of the underlaying soil upon wetting. Runoff and leakage of underground pipes, septic tanks and other water sources can endanger structures built on such problematic soils. During the recent years, different techniques have been proposed for reducing collapse potential to mitigate the adverse effects of such soils on engineering projects. In this study, the feasibility and efficacy of treating collapsible soils with the bacterium Bacillus sphaericus to reduce the soil collapsibility have been examined. To inspect the efficiency of this stabilisation technique and to explore different influential physical factors, collapsible soil samples have been prepared and treated with bacterial solutions of different bacterial density. The effect of curing time, amount of precipitating agent and bacterial density on the mechanical behaviour of collapsible soils have been studied and favourable conditions for soil improvement are presented and discussed. The maximum reduction in collapse potential achieved in these tests was about 65%. The improvement occurred mainly in the first four days of treatment and the results showed that the method of blending the bacterial solution has a strong effect on the results obtained. © The authors and ICE Publishing: All rights reserved 2015.