1. 1. The total free amino acid (FAA) level in Balanus balanoides varied seasonally with a maximum level in the autumn and a minimum level in the spring. 2. 2. Proline, glycine, taurine, arginine, alanine, glutamic acid and serine were the most prominent free amino acids. The level of glycine showed a marked increase in November at the time of fertilization. Alanine had its highest level during the winter when food is sparse and feeding rates are reduced. 3. 3. Barnacles kept without food at 5° and 15°C from October to December had an increased FAA content compared with animals on the shore. There was a marked increase in the alanine content of animals kept at 15°C. 4. 4. Barnacles kept without food at 5°C from January to June had a lower FAA content than animals on the shore. Animals kept at shore temperatures and fed Artemia nauplii from mid-February onwards had a similar FAA content to the shore animals. 5. 5. A survey of aminotransferase activity in B. balanoides showed strong glutamate-pyruvate transaminase (E.C. 2.6.1.2) activity but no glutamate-oxaloacetate transaminase (E.C. 2.6.1.1) activity. Aminotransferases involving the amination of glyoxylic acid to glycine were found to be active.