Use of the mycopathogen Beauveria bassiana (strain GHA), marketed as BotaniGard(®) ES, was evaluated as a plant protection strategy against the spider mite Tetranychus urticae Koch, which is considered one of the most economically important and cosmopolitan pests of many crops. Tetranychus urticae were treated with four concentrations of conidia (1 × 10(5), 1 × 10(6), 1 × 10(7), or 1 × 10(8) conidia/ml), and virulence was assessed on mites held at four relative humidity levels (35, 55, 75, and 95 ± 2 % RH) at 25 ± 1 °C. At 1 × 10(8) spores/ml, the LT50 value was 9.7 h at 95 % RH, which was significantly lower than values for other RH levels. At 1 × 10(7) spores/ml, the LT50 value was 43.8 h at 95 % RH, which was significantly different from values at 55 and 35 % RH. The efficacy of B. bassiana product was also verified on mites infesting potted bean plants with a concentration of 1 × 10(8) spores/ml. In double spray treatment where applications were made 2 × on days 5 and 10 after mite infestation, the nymphal and adult population of T. urticae were reduced to zero on days 20 and 15, respectively. With a single spray on day 5, the nymphal population was also greatly reduced, but increased rapidly after day 20. Single and double sprays with B. bassiana reduced leaf damage as measured by image analysis by 33 and 94 % compared to no treatment, respectively. These results suggest that 1 × 10(8) spores/ml was the most effective dose and that two applications, at a 5-day interval, provided control of T. urticae in our laboratory assay.