The purpose of this book is to illustrate a selection of biological prop erties of bacteria that reveal them as important living beings. We have primarily addressed readers who have had some previous educa tion in the natural sciences, and we have assumed a modest under standing of elementary chemical and biological principles. Our aim is to provide abrief survey of bacterial forms and structures, placing special emphasis on the activities of bacteria in their environment and some important interrelations within it. Bacterial ecobiology is the study of those aspects of bacteria that influence, and are in fluenced by, environmental phenomena. Some material traditionally covered in standard texts-such as medi cal bacteriology and immunology, applied bacteriology, and bacterial classification-will not be found here, because it is our opinion that these are peripheral to the idea of ecobiology and because numerous excellent treatments of this material are readily available. There is also no formal presentation ofbacterial genetics or of molecular biology per se in this book. However, mention of phenomena involved in these subjects is made where considered appropriate.