The Symposium on Swimming and Flying in Nature which was held at the California Institute of Technology, Pasadena, California from July 8-12, 1974 was conceived with the objective of providing an interdisciplinary forum for the discussion of funda mental biological and fluid mechanical aspects of these forms of natura110comotion. It was the earnest hope of all concerned in the organization of the Symposium that the exchange of knowledge and interaction of ideas from the disciplines involved would stimu late new research in this developing field. If the liveliness of the discussion generated among the 250 or so participants is any measure, then this objective was fulfilled to a significant degree. These two companion volumes contain the manuscripts of the papers presented during the Symposium. It is hoped that this permanent record will serve to perpetuate the enthusiasm and active thought generated during those days in Pasadena. The first volume contains the proceedings of the first two days of the confer ence (Sessions I to IV) which concentrated on the locomotion of micro-organisms. The second volume (Sessions V to VIII) deals with the propulsion of larger fish, insects and birds. Professor Sir James Lighthill's Special Invited Lecture which opened the Symposium is contained in the second volume, rather than the first, since it deals with natural flight.