Photon yields of oxygen evolution at saturating CO2 were determined for 44 species of vascular plants, representing widely diverse taxa, habitats, life forms and growth conditions. The photonyield values on the basis of absorbed light (φ (a)) were remarkably constant among plants possessing the same pathway of photosynthetic CO2 fixation, provided the plants had not been subjected to environmental stress. The mean φ (a) value ±SE for 37 C3 species was 0.106±0.001 O2·photon(-1). The five C4 species exhibited lower photon yields and greater variation than the C3 species (φ (a)=0.0692±0.004). The φ (a) values for the two Crassulaceanacid-metabolism species were similar to those of C3 species. Leaf chlorophyll content had little influence on φ (a) over the range found in normal, healthy leaves. Chlorophyll fluorescence characteristics at 77 K were determined for the same leaves as used for the photon-yield measurements. Considerable variation in fluorescence emission both at 692 nm and at 734 nm, was found 1) among the different species; 2) between the upper and lower surfaces of the same leaves; and 3) between sun and shade leaves of the same species. By contrast, the ratio of variable to maximum fluorescence emission at 692 nm (Fv/FM, 692) remained remarkably constant (The mean value for the C3 species was 0.832±0.004). High-light treatments of shade leaves resulted in a reduction in both φ (a) and the Fv/FM, 692 ratio. The extent of the reductions increased with time of exposure to bright light. A linear relationship was obtained when φ (a) was plotted against Fv/FM, 692. The results show that determinations of the photon yield of O2 evolution and the Fv/FM, 692 ratio can serve as excellent quantitative measures of photoinhibition of overall photosynthetic energy-conversion system and of photochemistry of photosystem II, respectively. This is especially valuable in field work where it is often impossible to obtain appropriate controls.