An eukaryotic alga (strain BAC 9708) from the picoplankton of Lake Baikal (Eastern Siberia) has been cultivated in order to investigate its morphology, ultrastructure, growth requirements, pigment composition and nuclear-encoded 18S and chloroplast-encoded 16S rDNA sequences. Cells of strain BAC 9708 contain a nucleus, a single chloroplast without a pyrenoid, one mitochondrion, a poorly visible dictyosome, a large vacuole and a cell wall with an outer sporopollenin-containing layer. Ellipsoidal cells, 1.5–2.0 × 1.0–1.5 μm in size, propagate usually by two, or infrequently by four autospores. Sequence analysis of nuclear-encoded rDNA showed that all 581 nucleotides of the 5′-end of the 18S rDNA were identical to that of Choricystis minor (Skuja) Fott (Chlorococcales, Chlorophyta) obtained earlier (Krienitz et al., 1996). The chloroplast-encoded rDNA sequence of this strain showed identity with one of three chloroplast sequences, selected by PCR-amplification with 16S rDNA specific primers from total picoplanktonic DNA, obtained previously.