Studies on the effects of crude oil and its fractions on the blood haemoglobin (HB) neutrophil (NP) concentration in the blood of Heterobranchus bidorsalis juveniles were carried out. Two study periods (toxicity and recovery) and four oil types (Bonny-light crude oil (BLCO), premium motor spirit (PMS), kerosene (DPK) and lubricating engine oil (lBO) at 1.00, 2.00, 4.00 and 8.00ml were used. There were significant differences (P<0.05, p< 0.01) on the HB and NP values in blood of fish samples. The elevated values of HB levels in fish exposed to 2.00,4.00 and 8.00m1 L-1 BLCC, 2.00ml L-1 PMS and DPK; and 4.00ml L -1 LBO over the levels in the control fish were in consonance with the results of other workers. Increases in NP values in fish exposed to the four oil types (BLCO, PMS, DKP and LBO) relative to the control could be part of the immunological attempts by the fish to mobilize the body's defence mechanisms. The relatively high percent mortality and low survival of H. bidorsalis juveniles exposed to 4.00 and 8.00mlL-1 concentration of the four oil types over the other concentration implied that these concentrations were probably lethal to fish existence
