Walter M. Fitch (May 21, 1929 – March 10, 2011) was professor of molecular evolution at the University of California, Irvine, until his death. He was also a member of the National Academy of Sciences, the American Philosophical Society, and the American Association for the Advancement of Science, and was a Foreign Member of the Linnean Society (London). He is the co-founder of the journal Molecular Biology and Evolution, together with Masatoshi Nei, and was the first president of the Society for Molecular Biology and Evolution.
Walter Fitch is noted for his pioneering work on reconstruction of phylogenies (evolutionary trees) from protein and DNA sequences. Among his achievements are the first major paper on distance matrix methods, which introduced the Fitch-Margoliash method which seeks the tree that best predicts a set of pairwise distances among species. He also developed the Fitch parsimony algorithm, which evaluates rapidly and exactly the minimum number of changes of state of a sequence on a given phylogeny. His definition of orthologous sequences has been very often cited and used as a reference in many research publications.