Algal transformation of cyclohexanone (C6ON) was photoautotrophically studied with an axenic strain of Chlorella pyrenoidosa Chick. The algal reduction of C6ON to cyclohexanol (C6OL) was confirmed by GC-MS analysis, but C6OL 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 C6ON to C6OL, although increasing C6ON concentrations resulted in a decrease in cell growth. The maximum accumulation of C6OL in growing culture was observed at 0.2% C6ON.