The lettuce aphid, Nasonovia ribisnigri (Mosley), is an economically important pest of lettuce worldwide. Recently, the entomopathogenic fungus, Beauveria bassiana strain GHA, and the aphelinid parasitoid, Aphelinus abdominalis Dalman, have been reported to be potential biological control candidates for use against N. ribisnigri. However, no information is available on the interaction between B. bassiana and A. abdominalis when both are applied. This study therefore examined the compatibility of B. bassiana and A. abdominalis in laboratory experiments. Specifically, we assessed the susceptibility of two A. abdominalis developmental stages (larvae and pupae) to two spore concentrations of B. bassiana (high: 1 x 10(9) and low: 1 x 10(4) conidia/ml) and a control of 0.01% Tween 80. We found parasitoid larvae to be highly susceptible to infection at the high spore concentration of B. bassiana, as measured by rates of mummy formation (mean +/- SE: 14%+/- 2.23) and adult emergence (mean +/- SE: 10%+/- 5.56) compared with the control treatment (mummification: mean +/- SE: 79%+/- 3.22; adult emergence: mean +/- SE: 87%+/- 4.40). In contrast, B. bassiana had no effects on parasitoid development when parasitoid larvae were treated with the lower spore concentration or parasitoid pupae were treated with either high or low spore concentrations. This study suggests that it might be possible to combine B. bassiana and A. abdominalis for integrated pest management of N. ribisnigri. As such, the application of B. bassiana should be timed to coincide with the presence of advanced developmental stages of A. abdominalis to protect the parasitoid. Another option would be to delay the release of A. abdominalis after B. bassiana application, when A. abdominalis is no longer susceptible to fungal infection.