This grant provides funds for the support of a Gordon Conference on the Population Biology and Evolution of Microorganisms, to be held in Plymouth, New Hampshire, July 24- 28, 1989. The field of microbial population and evolution is a growing field of research. It is an interdisciplinary field in which the power of the techniques of microbial genetics and molecular biology combined with increased emphasis on hypothesis testing in evolutionary biology has brought microorganisms into the mainstream of the fields of population genetics and evolutionary biology. In fact, microorganisms with their large populations, short generation times and biochemically defined genetics provide the only practical systems for experimental testing of some kinds of evolutionary mechanisms. Funds will be used to help cover expenses of speakers and discussants and to permit a modest number graduate students, post-doctoral fellows, and younger scientists whose resources are limited to attend the conference.