SYNOPSIS. Striking progress in our understanding of parasitic protoroa has been achieved thru the employment of advanced electronmicroscopic, biochemical, immunologic, and cultivation methods. Some recent information gathered by means of these methods on trichomonad and trypanosomalid flagellates as well a s on eimeriidian and plnsmotliid haemosporiidian Sporozoa is discussed. It is empha-T is evident from the recent reports published in a variety I of journals, review articles, and books that the most striking progress in our understanding of parasitic protozoa has been achieved thru the employment of advanced electronmicroscopic, biochemical, immunologic, and cultivation methods. Among the organisms about which much information has been accumulated by {these methods are the trypanosomatid and trichomonad flagellates and the plasmodiid haemosporidians. For this reason, and also because of my relatively greater familiarity with them, the following discussion will deal largely with these assemblages. The elimination of many other groups of parasitic protozoa from the present consideration must not be construed as reflecting lack of activity in their studies, but merely less activity and also poorer knowledge of them on my part.Several recent publications reviewing the parasitic protozoa (21,23,31,36,82,98) indicate (that data pertaining to biochemistry are based to a very significant degree upon studies of organisms grown in culture. Indeed, in many instances, analytic methods could not have been used without large yields of organisms achievable only through cultivation on nonliving media. The observations I propose to make concerning cultivation are relevant almost exclusively to extracellular species. Maintenance of intracellular forms in the presence of host cells or cell cultures, or even free of all such cells(for references see 36), has contributed much to our understanding of leishrnanias, and particularly of malarial parasites. These methods of cultivation involve somewhat different problems, which will not be considered here.