Mating affinity, nuclear rDNA internal transcribed spacer (ITS) sequence, and geographic distribution of more than 100 isolates of Pandorina, Volvulina, and Yamagishiella were determined. Comparative analysis of ITS sequences reaffirmed the unity of Pandorina morum and its separation from all other species of Volvocaceae except those of Volvulina. This latter genus, represented by four species, appears to represent only morphological variants of several P. morum subclades. The P. morum clade (including also additional species such as P. smithii and P. colemaniae) encompasses an evolutionary span, as determined by comparison of ITS, greater than either of the multispecies genera Gonium or Eudorina. There are at least 30 Pandorina/Volvulina syngens, sexually isolated groups, so far as can be determined, among the current collection of strains. In addition, as in other volvocacaean genera, two clones are homothallic, capable of forming zygotes within a genetic clone. The existence of so many syngens suggests that considerable evolutionary diversification of the genes controlling gamete compatibility and intercross survival has occurred, unaccompanied by significant morphological change. Within each syngen, genetic distance increases with geographic distance between collection sites. At least half of the isolates studied must have been introduced northward since the Pleistocene. Although we probably know more about characters in this group of algae than any other algal group, in part because they are so easy to culture, we are still largely ignorant of what circumscribes their niche in nature. The study of all these organisms, distributed throughout the world presumably by the activities of shorebirds, emphasizes the evolutionary role of mating genes in the inevitable formation of new genetically isolated subclades in these eukaryotes and provides initial data on their rate of appearance versus their rate of distribution over the earth.