Cercomonads are among the most abundant and widespread zooflagellates in soil and freshwater. We cultured 22 strains and report their complete 18S rRNA sequences and light microscopic morphology. Phylogenetic analysis of 51 Cercomonas rRNA genes shows in each previously identified major clade (A, B) two very robust, highly divergent, multi-species subclades (A1, A2; B1, B2). We studied kinetid ultrastructure of five clade A representatives by serial sections. All have two closely associated left ventral posterior microtubular roots, an anterior dorsal root, a microtubule-nucleating left anterior root, and a cone of microtubules passing to the nucleus. Anterior centrioles (=basal bodies, kinetosomes) of A1 have cartwheels; the posterior centriole does not, suggesting it is older, and implying flagellar transformation similar to other bikonts. Strain C-80 (subclade A2) differs greatly, having a dorsal posterior microtubule band, but lacking the A1-specific fibrillar striated root, nuclear extension to the centrioles, centriolar diaphragm, extrusomes; both mature centrioles lack cartwheels. For clade A2 we establish Eocercomonas gen. n., with type Eocercomonas ramosa sp. n., and for clade B1 Paracercomonas gen. n. (type Paracercomonas marina sp. n.). We establish Paracercomonas ekelundi sp. n. for culture SCCAP C1 and propose a Cercomonas longicauda neotype and Cercomonas (=Neocercomonas) jutlandica comb. n. and Paracercomonas (=Cercomonas) metabolica comb. n.