Technical Abstract: There seems to be a general consensus that entomopathogenic hyphomycetes will lose their ability to infect insects when routinely subcultured on artificial media. Published effects of repeated in vitro subculture on virulence and morphological characteristics of entomopathogenic fungi appear to vary considerably among isolates and species. In the current study we assessed effects of repeated in vitro passage on the virulence of the B. bassiana GHA strain, the active ingredient in the commercial products BotaniGard and Mycotrol (Mycotech Corp., Butte, MT). The fungi were assayed against neonate and 2-d old (pre-molt) Bemisia argentifolii ('whitefly') nymphs infesting bean leaves (var. Royal Burgundy) rooted in Oasis rooting cubes (Smithers-Oasis USA, Kent, OH). Two doses (4.0x107 and 2.5x106 viable conidia per ml of 0.02% Silwet L77) were used to enhance the possibility of detecting small changes in virulence that could be missed if only a single dose was used. No significant differences were detected between mortality and infection rates for fungi passaged 1, 5, 10 or 15 times at either test concentration. The results of the current study strongly indicate that the genetic factors controlling pathogenicity in the B. bassiana GHA strain are stable enough to facilitate limited repeated transfer and culture on artificial media, such as during the mass-production process, without affecting the virulence of the isolate.