The study was oarried out on three species of fish which are commerciallyimportant and presently cultivated in Sri Lanka. These are Oreochromismossambicus, Oreochromis niloticus and Chanos chanos. Blood parametersmeasured in the resting state for the three species were haematocrit, erythrocytecounts and haemoglobin concentrations. From these findings the followingwere calculated. The Hb content per single erythrocyte, the Hb concentration per single erythrocyte and volume of a single erythrocyte. It wasfound that all these values were higher in the Oreochromis species comparedto Chanos. Inverse relationships between erythrocyte count and volume ofa single erythrocyte and between cell count and Hb content per single erythrocyte were observed. O2 capacities and O2 dissociation curves were also determined. The effect of pH on 0. affinity was also studied. In vivo experimentswere conducted to measure rates of 02 consumption in the resting state of fish