Ultrastructural characteristics of merogony and the development of mature merozoits and macrogametogensis of Eimeria sp. infecting the Libyan jird were investigated. Mono and binucleated schizonts were detected. Developed merozoites showed all the apicomplexan architecture (Pellicle, conoid, rhoptries, micronemes … etc). Transformation into macrogametes were studied. Early macrogamonts characterized by the loss of all apicomblexan characters and the appearance of wall-forming bodies were the first indication for macrogametogenesis. As development precepded, two types of wall forming bodies (I, II) were clearly detected. Cell organelles including amylopectin granules and lipid globules were greatly increased in mature macrogametes. Young oocyst (zygote) with double-layer oocyst wall were also detected. Host cell reaction due to infection included hypertrophy of the infected host cell, enlargement, deformation and displacement of the host cell nucleus. Swollen and degeneration of mitochondria, endoplasmic reticulum and cytoplasmic vacuolation of the host cell. All parasite stages were enclosed in parasitophorous vacuole limited by unit membrane. Extended damage effect appeared in some neighboring host cell indicating the secretion of some toxins by the parasite.