Algal transformation of cyclohexanone (C 6 ON) was photoautotrophically studied with an axenic strain of Chlorella pyrenoidosa Chick. The algal reduction of C 6 ON to cyclohexanol (C 6 OL) was confirmed by GC-MS analysis, but C 6 OL could not be further transformed by this strain. Three cycloalkanones, cyclopentanone, cycloheptanone, and cyclooctanone, were also converted to their corresponding alcohols in a growing culture of the same strain. The pH of the medium did not influence the algal reduction of C 6 ON to C 6 OL, although increasing C 6 ON concentrations resulted in a decrease in cell growth. The maximum accumulation of C 6 OL in growing culture was observed at 0.2% C 6 ON.