Advances in Microbial Ecology was established by the International Com­ mittee on Microbial Ecology (ICOME) as a vehicle for the publication of critical reviews selected to reflect current trends in the ever-expanding field of microbial ecology. Most of the chapters found in Advances in Microbial Ecology have been solicited by the Editorial Board. Individuals are encouraged, however, to submit outlines of unsolicited contributions to any member of the Editorial Board for consideration for inclusion in a subsequent volume of Advances. Contributions are expected to be in­ depth, even provocative, reviews of topical interest relating to the ecology of microorganisms. With the publication of Volume 8 of Advances we welcome to the panel of contributors Martin Alexander, the founding editor of this series, who discusses the range of natural constraints on nitrogen fixation in agri­ cultural ecosystems. Ecological aspects of cellulose degradation are discussed by L. G. Ljungdahl and K. -E. Eriksson, and of heavy metal responses in microorganisms by T. Duxbury. In his chapter, A. Lee con­ siders the gastrointestinal tract as an ecological system, and comments on the possibility of manipulating this system. The complex interactions among aerobic and anaerobic sulfur-oxidizing bacteria are discussed in terms of natural habitats and chemostat culture by J. G. Kuenen, L. Rob­ ertson, and H. van Gemerden. Finally, J. A. Robinson presents the advantages and limitations in the use of nonlinear regression analysis in determining microbial kinetic parameters in ecological situations. K. C. Marshall, Editor R. M. Atlas B. B.