We report the complete DNA sequence of the short repeat region in the genome of herpes simplex virus type 1, as 6633 base pairs of composition 79.5% G+C. This contains immediate early gene 3, encoding the IE175 protein, an important transcriptional activator of later virus genes. The IE175 coding region was identified as a 3894 base sequence of 81.5% G+C DNA. The base composition of this gene is thus the most extreme yet determined, and the IE175 predicted amino acid composition is correspondingly biased, most notably with an alanine content of 20.9%. Functionally important regions of the IE175 polypeptide were tentatively identified by comparison with the sequence of the homologous protein from varicella-zoster virus and from locations of ts mutations, and were correlated with properties of the amino acid sequence. Aspects of the evolution of such an extreme composition DNA sequence were discussed.