The comparative growth and efficacy of three strains of Trinidadian Isaria fumosorosea Wize (T,T10 and T11) blastospores were assessed as spray suspensions for controlling greenhouse whitefly, Trialeurodes vaporariorum Westwood. Percent germination of all the three strains were unaffected by the surfactants Triton X -100 or Tween 80 compared to the control (water). Radial growth of fungal strain T11 was faster than either T or T10 which were similar in their growth patterns. Virulence of blastospores of three strains were tested against pharate adult greenhouse whiteflies on bean plants (Phaseolus vulgaris L.). The corrected mortality of whitefly for strains T, T10 and T11 was 56, 68 and 98% at 107  blastospores mL-1 and the median lethal concentration observed was 2.1 x 107 , 1.3 x 107 and 2.5 x 106 blastospores mL-1, respectively. Strain T11 was the most virulent (range: 97-98% mortality) and had the lowest LC50 (range: 1.1-2.5 x 10  viable blastospores mL-1) value of the Trinidadian blastospore treatments tested against greenhouse whitefly on P. vulgaris.