ABSTRACTCrypthecodinium cohnii, a small marine heterotrophic homothallic dinoflagellate, has diversified into a complex of morphologically very similar breeding groups (biological species or sibling species), some of which have become widely dispersed. Membership of two clones in the same sibling species is shown by their sexual compatibility as determined by genetic complementation in zygotes formed from motility mutants derived from the two stocks. Membership in different sibling species may be inferrec when motility mutants of one strain do not complement those of another. Fifty‐six clones representing seaweed enrichments from *** geographic sites have been found to belong to 28 sibling species; 35 clones are members of seven wide‐ranging biological species, and 21 are single representatives of 21 other breeding groups within the ranges of the others. Of 174 clonal isolates in our possession, 168 conform in size and shape toC. cohnii.Six others which have smaller cells and only one‐fifth the standard DNA and chromosome number belong, we believe, to another species. TheC. cohniicomplex provides a unique opportunity for the study of evolutionary divergence and geographical dispersion of a dinoflage