A comparative study of chlorophyll (Chl) fluorescence characteristics was made between a soybean [Glycine max (L.) Merr.] Chl-deficient mutant (light green leaf, LG, Eji^ii), and a nearly isogenic Chl normál biotype (dark green leaf, DG, EnEn) of cultivar Clark, LG had a lower light-saturated net photosynthetic rate (/^n), compared with DG at early stages of growth (before pod filling), but the difference in Ejsi between LG and DG became smaller at later stages of growth. The lower of LG may result from lower Chl content (Chl a and b), lower carboxylation efficiency, and lower photochemical efficiency of photosystem 2. Both LG and DG grown at low iradiance [about 200-300 pmol(photon) m'^ s"’] in a growth cabinet had similar photochemical effíciencies. However, the efficiency was lower in LG than in DG grown in field sunlight in mid summer, especially during the aftemoon on clear days, indicating that LG is more sensitive to photoinhibition.