Abstract 1. 1. The purine nucleotides were determined in the whole blood of 13 species of seawater teleosts and 3 species of selachians. 2. 2. The ATP/ADP ratio ranged from 4 to 50 in the erythrocytes of the fish examined. 3. 3. GTP is widely distributed in fish erythrocytes but its level ranged from 2 to 27 nmol/mg Hb, the highest value being observed in Conger. 4. 4. Conger, Melanogrammus and Merlangus exhibited a GTP/ATP ratio higher than 1. The concentration of GTP per mol of Hb (physiologically most indicative) is higher than 1 in Conger, Melangrammus, Merlangus and Pollachius, and close to 1 in Molva and Gadus.