The effects of acclimation to captivity on blood composition of Nile tilapia Oreochromis niloticus were investigated. A totalof ninety fish, comprising forty five each of juvenile (mean length 10.22cm ±0.26SD; mean weight 54.62g±6.74SD) andadult (mean length 19.4cm±7.21SD; mean weight 346.22g±2041SD) were harvested from a reservoir at African RegionalAquaculture Centre, Aluu, Port Harcourt and acclimated in the experimental tanks for a period of seven days. Beforeacclimation significant difference (p < 0.05) were observed between the blood parameters of adult and juvenile fish with thefemale consistently having higher values than the males in all the parameters. After acclimation to captivity, there wassignificant reduction (p<0.05) in the values of haemoglobin (Hb), packed cell volume (PCV), red blood cell (RBC), meancorpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), lymphocytes and thrombocyte,while the values of mean corpuscular volume (MCV), white blood cell (WBC), neutrophils (NEUT) and monocytes (MON)increased significantly (p < 0.05). Results from this study therefore suggest that sex and size have some degree of influenceon the blood characteristics of O. niloticus.