The symposium, Microbial Diversity in Time and Space, was held in the Sanjo Conference Hall, University of Tokyo, Tokyo, Japan, October 24-26, 1994. The symposium was organized under the auspices of the Japanese Society of Microbial Ecology and co-sponsored by the International Union of Biological Sciences (IUBS), International Union of Microbiological Societies (IUMS), International Committee on Microbial Ecology (ICOME), and the Japanese Society of Ecology. The aim of the symposium was to stress the importance of the global role of microorganisms in developing and maintaining biodiversity. Twenty-four speakers from seven countries presented papers in the symposium and in the workshop, Microbial Diversity and Cycling of Bioelements, that followed the symposium. Papers presented at the symposium are published in this proceedings. Discussions of the workshop, which were energetic and enthusiastic, are also summarized in this proceedings. The symposium provided an opportunity to address the role of microorganisms in global cycles and as the basic support ofbiodiversity on the planet. Previously unrecognized as both contributing to and sustaining biodiversity, microorganisms are now considered to be primary elements of, and a driving force in, biodiversity. Financial support was provided for the symposium by the CIBA GEIGY Foundation for the Promotion of Science, Naito Foundation, and the Uchida Foundation of the Ocean Research Institute, University of Tokyo. Support from these foundations is gratefully acknowledged. CONTENTS Microbial Biodiversity-Global Aspects ................................. 1 Rita R. Colwell 2. Importance of Community Relationships in Biodiversity ...................