ABSTRACT. Biocontrol of the whitefly Trialeurodes vaporariorum (Westwood) (Hemiptera:Aleyrodidae) using entomopathogenic fungi has been a difficult challenge under greenhouse conditions. In order to select fungal isolates adapted to high temperature and extremely low moisture nine isolates of Lecanicillium lecanii (Zimmerman) Zare & W. Gams, L. attenuatum Zare & W. Gams and L. longisporum (Petch) Zare & W. Gams (Hypocreales:Clavicipitaceae) were evaluated. In vitro assays were performed to determine colony radial growth, conidial production and conidial germination in three water activity media (aw = 0.97, 0.98 and 1.00) at 28 and 32 °C. Virulence of Lecanicillium spp. isolates was evaluated against third instar T. vaporariorum on tomato plants at 23 °C. Colony radial growth, conidial production and germination decreased with the reduction in water activity, while 32 °C was extremely detrimental for all fungal isolates. However, some isolates were able to grow and produce conidia at low water activity and high temperature. Additionally, mortality above 60 % was recorded for one of these isolates. Practical implementation of biocontrol of T. vaporariorum under greenhouse production systems should consider the selection of those Lecanicillium isolates that show tolerance to the adverse environmental conditions in greenhouses. © 2013 International Organization for Biological Control (IOBC).