In the traditional sense, several families of Chlorococcales sensu lato, such as Golenkiniaceae, Micractiniaceae, and Scenedesmaceae contained taxa with bristle formation, whereas the Chlorellaceae contained only genera without bristles. However, phylogenetic analyses of the small subunit (SSU) and internal transcribed spacer (ITS) rDNA sequences have shown that the genera Micractinium, Diacanthos (formerly Micractiniaceae) and Didymogenes (formerly Scenedesmaceae) are closely related to the genus Chlorella. The bristle formation within the Chlorella-clade is originated independently in four lineages: Micractinium pusillum, Diacanthos belenophorus, Didymogenes anomala, and Micractinium parvulum (also known as Golenkinia minutissima). The latter species is to exclude from the genus Micractinium. Consequently, we proposed the new genus Hegewaldia and transferred M. parvulum to this genus. In contrast, Diacanthos belenophorus is closely related to Micractinium pusillum. As a result, the new combination (Micractinium belenophorum comb. nov.) is proposed. Comparisons of the secondary structure of ITS-1 and ITS-2 rDNA sequences among the strains of Didymogenes and Hegewaldia are provided to support the species concept in these genera.