Light and electron microscopy have been used to study blood stages of H. mehlhorni as well as different developmental stages of the parasite within lung tissues of the naturally infected vipers Echis carinats captured from Siwah Oases, Egypt. A natural infection rate of 60% was recorded among vipers. Two types of meronts were observed within the endothelial cells of the viper's lung. The first type is the small one, produced 6-15 merozoites. The second type is the large one, produced 20-40 merozoites. In both types of meronts, merozoites were developed through an ectomerogenous manner. The erythrocytic parasites and the merozoites within the endothelial cells of the viper's lung sharing all general architecture of the apicomplexa. Moreover, evident peculiarities of haemogregarines were recorded, such as the presence of micronemes and rhoptries on both sides of the parasite nucleus, and the presence of large number (up to 100) of subpellicular microtubules and micronemes (up to 150).