Hewitt et al. reported the detection of circadian control of isoprene emissions from two tropical rainforests in Malaysia, based on their nding that a model without a circadian control cannot reproduce the observations. By including circadian-controlled isoprene emissions into models of atmospheric chemistry and transport, they suggested that plant circadian rhythms indirectly aect the global concentration of surface-level ozone. Here, we argue that the circadian rhythm postulated by Hewitt et al is not robust, and depends on untested assumptions regarding both the temperature and light response of isoprene emissions, and the unaccounted-for eects of canopy structure. The apparent circadian control disappears if dierent, biologically realistic, model parameters are used.