This study aimed to evaluate the susceptibility of two cereal aphids, Sipha maydis (Passerini) and Metopolophium dirhodum (Walker), to the entomopathogenic fungus, Lecanicillium longisporum (Zimm.) Zare and Gams strain LRC 190, under controlled conditions. The conidial suspension of the fungus was administered using a sprayer on the whole plant over apterous adult aphids. The results indicated that both aphid species were susceptible to L. longisporum and that aphid populations were significantly reduced, compared to the control. Nine days after treatment, the LC50 value of the fungus was obtained as 5.9 × 105 and 3.2 × 106 conidia/ml for S. maydis and M. dirhodum, respectively. The LT50 value of the fungus at a concentration of 108 conidia/ml was obtained as 2.9 and 4.4 days for S. maydis and M. dirhodum, respectively. The results demonstrated that there was a varying susceptibility to the fungus between aphid species. The estimated LC50 and LT50 indicated that L. longisporum was more virulent to S. maydis than to M. dirhodum. The LT50 and R0 decreased as the conidial concentration increased. This is the first study to demonstrate the susceptibility of S. maydis to the entomopathogenic fungi. The present study suggests that L. longisporum has high virulence against the aphids S. maydis and M. dirhodum. Further research with an emphasis on greenhouse and field tests are required, however, before making any decision about using the fungus in a control program