Measurements of nonmethane hydrocarbon concentrations and gradients above Harvard Forest (42°32′ N, 72°11′ W) are reported for January through December 1993, along with inferred whole-ecosystem emission rates for ethene, propene, and 1-butene. Emissions were calculated using a micrometeorological technique where the ratio of observed CO2 fluxes and gradients were multiplied by the observed hydrocarbon gradients. Average emissions of ethene, propene, and 1-butene during summer were 2.63, 1.13, and 0.41 × 1010 molecules cm−2 s−1, respectively. Emission of these olefins was correlated with incident solar radiation, implying a source associated with photosynthesis. In the northeastern United States, summertime biogenic emissions of propene and 1-butene exceed anthropogenic emissions, and biogenic emissions of ethene contribute approximately 50% of anthropogenic sources. Our measurements suggest that terrestrial biogenic emissions of C2-C4 olefins may be significant for atmospheric photochemistry.