Actinastrum hantzschii LAGERHEIM is characterized by spindle-shaped cells that are truncated at one end and arranged 3-dimensionally in colonies. An isolate of the alga was characterized by the use of light and electron microscopy as well as 18S rRNA gene sequencing. Despite morphological differences, phylogenetic analyses indicated that A. hantzschii is closely related to the trebouxiophycean taxa Chlorella sorokiniana SHI-HIRA et KRAUS, C. vulgaris BEIJERINCK and C. lobophora ANDREYEVA, and did not show any affinity to the Scenedesmaceae as previously proposed. Previous studies on Closteriopsis acicularis G. M. SMITH (Trebouxiophyceae), together with the findings of the present study, question the concept of Chlorella sensu stricto as well as the criteria currently used in the systematics of coccoid green algae.