The potential for biological control of the hop aphid using an aphid-derived isolate ofBeauveria bassianawas investigated in the laboratory and field. In initial bioassays using conidial suspensions at dosages ranging from 104to 108a conidia/ml, the isolate exhibited a high level of virulence toPhorodon humuli(Schrank), yielding a LC50of 1.37 × 105conidia/ml. LT50values decreased with increasing conidial dose, being 11.1 ± 4.2, 8.4 ± 4.0, 5.0 ± 1.5, 3.6 ± 1.5, and 3.1 ± 1.1 d at 104, 105, 106, 107, and 108conidia/ml, respectively. A potential for long-term control was demonstrated when conidial suspensions were applied to aphid-infested potted hop plants. Aphids increased only slightly during a 4-wk period on potted hop plants treated with a suspension of 107 conidia/ml. This was significantly lower than the net increase found on the controls and on plants treated with a suspension of 105conidia/ml. However, there was no mycosis detected in aphids when a 107 conidia/ml suspension was applied in an aphid-infested commercial hop yard.