The entomopathogenic fungus Alternaria alternata grew equally well on potato dextrose agar (PDA) and also on potato extract agar (PEA). The latter was excellent for sporulation. The fungus grew at a wide temperature range of 15–35 °C with an optimum at 25 °. On PDA sporulation was much reduced as compared to that on PEA. The spores of the fungus germinated only at 100% relative humidity (rh). Darkness significantly increased the germination percentages in the early stages of germination at all temperatures tested. Differences in germination percentages between light and dark conditions were mostly levelled 24 h post incubation. The germ tubes formed distinct appressoria, both on glass slides and on the aphid host Anuraphis nerii, on which appressoria were formed on all parts of the insect bodies, even on the hardest ones, such as the head and thorax. Most appressoria were single globose and formed terminally on hyphal branches. Few were subapical and still fewer compound and multilobed. Detachment of appressoria from their original position revealed an exceedingly smoothened appression area, around which a mucilagenous material was secreted, apparently cementing them onto the host epicuticle. Optimum temperature for appressorium formation was 20–25 °. At these temperatures more than 80% of the emerging germ tubes formed appressoria. The factors affecting spore germination and appressorium formation are discussed in relation to the prevailing environmental conditions in the mediterranean area as well as their role in influencing the spread and activities of the pathogen and the infection of aphids under field conditions.