We used a small oxygen electrode and a grating monochromator of 10 mμ half-band width to determine light-saturation curves of photosynthesis for films of Chlorella pyrenoidosa no more than 1 cell thick. All curves were referenced to the lightsaturated rate observed in 5 mw/cm2 of 680 mμ. To a close approximation (± 2%) the light-saturated rate was independent of wavelength over the region in which light intensity was sufficient to make the test (450-705 mμ).