Abstract
The use of triplet code words inE. coli,ФX174, MS2, and rabbit globin was examined. A significant deficiency of purines in the third position of fourfold degenerate codons was noted, although its significance is not understood. There has been no consistent selection against uracil in pyrimidine restricted codons. For many amino acids the choice between code words appears random, while for arginine, isoleucine, and probably glycine, distinct biases exist which can be explained in terms of tRNA availability.