Toxicity of the organophosphate, diazinon was studied in grass carp (Cetenopharyngodon idella) weighing 5±1.0g, under static water quality condition at 16°C. Also, some hematological and biochemical variables of fish weighing 50±5.0g were studied 12 hours post-exposing fish to the toxicant at sublethal concentration of 5.6mg/L at 16°C. Values of 18.19, 17.21, 16.68, and 15.13mg/L, were obtained as LC50 after 24, 48, 72 and 96 hours post-exposing fish to the toxicant, respectively. Levels of red blood cells (RBC), hematocrit (PCV), hemoglobin (Hb), mean corpuscular hemoglobin concentration (MCHC), leukocyte count (WBC), lymphocyte and monocyte were significantly lower in the exposed fish than control fish (p<0.05), whilst values of polymorphonuclear cells (PMN) and myelocytes counts were higher in the test group (p<0.05). In addition, the levels of aspartate aminotransferase (AST), alkaline phosphatase (ALP) and lactate dehydrogenase (LDH) were significantly lower in fish exposed to the toxicant (p<0.05), while the level of alanine aminotransferase (ALT) was higher in experimental fish (p<0.05) than control fish. The levels of glucose and triglyceride were higher in fish exposed to toxicant than control fish. However, such differences were only significant for glucose level (p<0.05). In contrast, the levels of total protein and cholesterol decreased insignificantly in fish exposed to the toxicant compared to those in control fish (p>0.05). The obtained results show that although diazinon can be classified as a slight toxic chemical for grass carp, the toxicant negatively affects some immunophysiological functions of the fish including immunocompent cells.