Complete nuclear 18S rRNA and chloroplast 16S rRNA coding sequences of the fusiform green alga Closteriopsis acicularis (G. M. Smith) Belcher et Swale (Chlorophyta) were used to determine its phylogenetic position within the Chlorophyta. Phylogenetic trees inferred from both genes show that C. acicularis is most closely related to Chlorella kessleri Fott et Nováková (Trebouxiophyceae) and does not belong to the family Selenastraceae (Chlorophyceae) as proposed previously. Despite its morphological distinctness, C. acicularis has to be placed close to the genus Chlorella Beijerinck, and especially to C. kessleri. The chloroplast 16S rRNAs of C. acicularis and C. kessleri possess a unique insertion resulting in an additional AT-rich helix in a length-conserved region of the gene. A close relationship to the genus Chlorella is further confirmed by the pyrenoid ultrastructure. The 16S rRNA gene of C. acicularis contains a group I subgroup IA3 intron which is most similar to the first intron in the corresponding gene of Chlamydomonas pallidostigmatica King (Chlorophyta). In helix P6, the intron carries an additional extension related to intron-encoded endonucleases of the LAGLIDADG family. It is the first group I intron reported in chloroplast 16S rDNA of a species other than Chlamydomonas.