A review of the book, The Mycoplasmatales and the L-phase of Bacteria by Leonard Hayflick, Ed. is presented. That organisms which lack cell walls are under intensive study is evident from the table of contents and the size of this book. The editor has drawn together 26 chapters by 35 contributors, mostly from the Second International Conference on the Biology of the Mycoplasmas held in 1966. Fifteen of the chapters are updated and expanded versions of papers published earlier in the 871-page proceedings of the 1966 conference (Biology of the Mycoplasmas, Annals of the New York Academy of Sciences, vol. 143, 1967). The articles are well organized, clearly written, thoroughly referenced, and comprehensive. The book is not abundantly illustrated, but does contain sparkling photographs of mycoplasma organisms as viewed under the electron microscope. The book, covers topics which range from studies of ultrastructure to the epidemiology of Mycoplasma pneumoniae.