The Pacific Symposium on Biocomputing (PSB 2003) is an international, multidisciplinary conference for the presentation and discussion of current research in the theory and application of computational methods in problems of biological significance. The papers and presentations are collected in this archival proceedings volume. PSB 2003 brings together top researchers from the US, the Asia-Pacific region and around the world to exchange research findings and address open issues in all aspects of computational biology. PSB is a forum for the presentation of work in databases, algorithms, interfaces, visualization, modelling and other computational methods, as applied to biological problems, with emphasis on applications in data-rich areas of molecular biology.