This book is written for ecologists and protozoologists. Ecologists who study environments and biotic communities in which protozoa are im portant should find this book especially useful. During the last decade it has become clear that protozoa play important roles in natural eco systems, but few ecologists have a feeling for the functional properties and the diversity of these organisms. Protozoa pose or exemplify many general problems of population and community ecology, and of evo lutionary biology. In most respects the general ecological properties of protozoa are not fundamentally different from those of larger organisms; yet, due to their small size, short generation times, and ubiquitous oc currence they often present ecological phenomena in a new and dif ferent light. To this should be added that protozoa are well-suited for experimental work. Despite these advantages, the study of protozoa has played a relatively modest role in the development of ecology and ev olutionary biology, primarily, I believe, because most ecologists are unfamiliar with these organisms. I hope this book will attract more attention to these favorable characteristics of protozoa. I also hope that this book may make protozoologists aware of new aspects of their pet organisms. For a long time (that is, until the fun damental distinction between prokaryotic and eukaryotic cells was rec ognized) protozoa were believed to represent the simplest form of life. They were therefore extensively used for the experimental study of basic questions of cell biology.