1. 1. The temperature dependency of protein synthesis was studied in vivo in five species of Pacific fish collected in the Galapagos and Perlas Islands: batfish ( Ogcocephalus darwini), groupers ( Epinephelus labriformis), catfish ( Netuma platypogan), puffers ( Arothron hispidus) and triggerfish ( Suffiamen verres). 2. 2. Liver protein synthesis, assayed by a rapid pulse injection technique, showed a moderate temperature dependency ( Q 10 = 2–3 ) in the 15–30°C range for all species except puffers ( Q 10 = 10–20 ). Synthesis was inhibited above 32°C. 3. 3. Protein synthesis in triggerfish was measured by the constant-infusion technique. Synthetic rates (% of tissue protein synthesized per day) at 25°C were 20% for liver, 10% for gill, 1.8% for red muscle and 0.6% for white muscle. Q 10 in the 20°–30°C range was 3.0 ± 0.5° C for all tissues.