The effect of purified host-specific toxin from Bipolaris (Helminthosporium) maydis, race T, on dark or light-dependent CO2 fixation was studied with thin (1 × 8 mm) corn (Zea mays L.) leaf slices supplied H14CO3−. At 5 to 30 nanograms per milliliter (≅5 nanomolar), toxin significantly inhibited (20 to 40%) dark CO2 fixation in susceptible (T) corn slices after either dark or light preincubations of 10-20 minutes. The same concentrations were effective to the same degree on photosynthesis, but the effect differed in that significant inhibition occurred after 25 minutes and only with light preincubation. Light preincubation without toxin did not shorten the time required for inhibition of photosynthesis after addition of toxin. Once photosynthetic inhibition was entrained, it was not reversed by subsequent periods of darkness. The results suggest the possibility that race T toxin affects two separate metabolic sites, and the data are discussed in view of currently held concepts of toxin action in susceptible tissue.