AbstractLiving organisms are usually exposed to combinations of a number of physical and chemical environmental factors in their natural habitats. Pre-exposure to sub lethal doses of a chemical may induce enhanced tolerance to higher doses of the chemical during successive exposures. Pre-exposure may also make organisms more sensitive to later exposures to smaller doses of the chemical. Toxicity of chemicals is also significantly affected by physical environmental variables. Temperature is among the most important physical environmental factor that interact with toxic chemical substances affecting their toxicity. Due to these interactions, results from controlled laboratory experiments that consider a single factor cannot represent the true effect of the toxicant on an organism in its natural environment. Hence, for better safety recommendations, environmental risk assessment of toxic chemicals should consider as much environmental factors as possible.This study evaluates toxicity of thiacloprid to F. candida under two conditions. The first experiment evaluates toxicity of thiacloprid following pre-exposure (acclimation) to sub lethal concentration of the chemical. The second experiment assesses development of cross tolerance by F. candida to thiacloprid when combined with cold stress. In the pre-acclimation experiment, F. candida were exposed to sub lethal doses for two weeks before they are subject to higher doses. The cross tolerance experiments on the other hand evaluate toxicity of thiacloprid when combined with cold stress. The experiments were conducted by combining the two stress factors in three different orders (1. Cold and thiacloprid applied simultaneously, 2. Thiacloprid prior to cold, 3. Cold prior to thiacloprid).Pre-exposure had significantly reduced range of tolerance of F. candida to thiacloprid. ANOVA showed that pre-exposure reduced the concentration that kill 50 % of the population by a factor of five. Reproduction was also significantly reduced following pre-exposure. On the other experiment, cold stress and thiacloprid did not show development of cross tolerance. However, results from simultaneous exposure indicate that cold temperatures reduced toxicity of thiacloprid. Acute LC50 of thiacloprid during simultaneous exposure was 7 times less at 7 0C than the LC50 at 15 0C. However, cold stress applied either prior or following to thiacloprid did not show any effect on toxicity of thiacloprid. Adverse effect of thiacolprid on reproduction of F. candiad was also significantly aggravated by pre-exposure and cold stress.