Abstract: An outbreak of coccidiosis in laboratory rats has been described in this study. Clinically, out of 50 rats, 30 were died after showing symptoms of anorexia, emaciation and diarrhea. Post mortem examination showed the large and small intestine distended with necrotic and sloughed off mucosal epithelial cells with intestinal content. The examination of the gut contents revealed numerous Eimeria oocysts. After sporulation, the oocysts were identified as those of Eimeria nieschulzi. The infection due to Eimeria nieschulzi is confirmed by Polymerase Chain Reaction (PCR) after amplifying partial (~1.6 kb) nuclear 18S rDNA from DNA of E. nieschulzi. The histopathological examination showed severe damage of the lamina propria of the intestinal mucosa with numerous coccidian developmental stages in the epithelium of small intestine. Biochemical analysis of serum from infested rats presented a significant increase in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST), glucose, total proteins, globulin and inorganic phosphorous while a decrease in alkaline phosphatase and blood urea nitrogen with insignificant changes in potassium, sodium, magnesium and chloride.