AbstractThe effect of light on the herbicidal acitivity of paraquat on the tropical perennial weedpaspalum conjugatumhas been studied in rubber and oil‐palm plantations in Malaya. Results showed that paraquat (at 0.5 ib/acre; gave a more rapid desiccation when applied in full sunlight but gave more persistent control when applied under the 70% shade of older trees or in the evening compared with other times of day. A simlar two‐fold improvement in the duration of weed control was achieved by the addition of the photosynthetic inhibitor bromacil (at 0.25 ib/acre) to paraquat. The response to the mixture suggested a synergistic reaction between the two compou