The virulence of five Verticillium lecanii strains against greenhouse whiteflies, Trialeurodes vaporariorum, was tested at various temperatures as a major environmental factor. Strain CS-626 was found to be the most durable strain at a broad temperature range, and highly virulent against greenhouse whiteflies in a detached-leaf bioassay. In a tomato plant pot experiment, the $LT_{50};and;LC_{50}$ of the CS-626 strain were 6.2 days and $2.3{ imes}10^6$ conidia/ml, respectively. The optimal concentration of CS-626 for successful infection was $1{ imes}10^8$conidia/ml. These results indicate that the CS-626 strain of Verticillium lecanii has a strong potential for effectively controlling greenhouse whiteflies.