Measurements of isoprene concentration and flux were made at a mixed deciduous forest in southern Canada during 1995 to characterize diel and seasonal emissions and thus deduce annual inventories. Isoprene inventories are necessary for inputs to modeling systems to study atmospheric chemistry and carbon budgets. Despite adequate environmental conditions to promote emissions, the onset of isoprene emission occurred two weeks after full leaf expansion, and two additional weeks were required for plants to emit isoprene at the maximum capacity. Such maximum isoprene emission was measured during July when canopy isoprene fluxes reached 40–60 nmol (isoprene)·m−2 (ground area)·s−1. Isoprene emission precipitously declined in concert with autumnal leaf senescence, with fluxes reaching the detection limit before the forest became leafless. In addition to plant development controls on emissions, temperatures below 10°C strongly modulated isoprene emission. After plants were exposed to low temperatures, isoprene emission remained suppressed and did not correspondingly increase in the manner that temperature is known to influence isoprene biosynthesis. Using a one-dimensional model to vertically adjust temperature and visible solar radiation with depth in the canopy, coupled with a seasonally adjusted emission rate, we estimated that the forest produced 71 mmol isoprene/m2 during 1995. For a deciduous forest with final leaf-area index of 4.1 and active isoprene biomass of 75 g (dry mass)/m2, on average such isoprene source accounted for 2% of the carbon fixed through photosynthesis. The percentage of carbon entering the atmosphere in the form of isoprene became as high as 10% during warm (>30°C) and dry conditions. The data set reported here demonstrates that constant emission rates are inadequate to characterize emission rates for the entire growing season. Improved isoprene-emission inventories can be achieved if emission factors are seasonally adjusted. In this study we adopted a method to express the emission rates as a function of degree days.