An alga known as “Nannochloropsis”, isolated from a prawn farm in Hainan, China, has been critically investigated and identified as Chlorella, a member of the Chlorophyceae based on fatty acid composition, ultrastructure, and 18S rDNA. Cells of this alga were spherical, measured by 1–6 μm in diameter and were enclosed in thin walls of approximately 0.04 μm thickness. They contained several small mitochondria, two to three thylakoids and had no vacuoles. There were many pyrenoids in the algal cells and their thylakoid lamellae were sparse and not translucent. Many lipid droplets were present in the cytoplasm. The total lipid content of this alga was 3% per gram dry weight and its major fatty acids were C16:0, C18:0,C18:1, C18:2, C18:3and C20:0. Eicosapentaenoic acid (C20:5, EPA) was not detected. The length of its 18S rDNA sequence was 1,712 bp. 18S rDNA sequence analyses indicated that this alga was a species of Chlorella.