Hematology and serum chemistry reference values were determined for cultured hybrid tilapia (Oreochromis hybrid). Fish (220 g.) were cultured in eight high density production tanks and water was recirculated over a rotating biologic contactor. Water quality, temperature, and diet were similar in all tanks. Ten fish from each tank were sampled over a 16 day period. The hematological values measured were total red cell count, packed cell volume, total hemoglobin, MCV, MCH, MCHC, plasma protein, total white cell count, and a white cell differential. The white cells counted were identified as small lymphocytes, large lymphocytes, neutrophils, monocytes, eosinophils, thrombocyte- like cells (TLC), and thrombocytes. Serum chemistry values included total protein, albumin, globulin, ammonia, creatinine, alkaline phosphatase, aspartate aminotransferase, total bilirubin, cholesterol, glucose, calcium, phosphorus, magnesium, sodium, potassium, and chloride. Reference intervals were determined following guidelines set by the National Committee on Clinical Laboratory Standards. Values obtained from these fish are similar to values published for other Tilapiine species. The reference intervals determined from this study will establish an initial baseline that will be useful in monitoring the health status of tilapia in high density recirculation systems.