Tropospheric ozone is formed by photochemical reactions between nitrogen oxides (NOx) and volatile organic compounds. In some regions, biogenic isoprene may be a significant contributor to the production of tropospheric ozone. The contribution of biogenic isoprene is an important aspect of regional ozone chemistry as it represents an ozone precursor that cannot be eliminated through emissions controls. The purpose of this study was to evaluate the contribution of isoprene to the production of tropospheric ozone in the Lower Fraser Valley, British Columbia. Seasonal trends and diurnal profiles were used to examine isoprene's relationship with temperature, to determine its source, and to investigate the chemical and physical factors that limit the ambient levels of isoprene present in the region. Total isoprene levels in the Lower Fraser Valley were low, and evidence suggested that a substantial fraction originated from anthropogenic rather than biogenic sources. Diurnal isoprene profiles were generally flat, and the times of the highest concentrations did not coincide with peak NOx levels nor with the times of optimal ozone-producing meteorological conditions. These results are consistent with those of previously reported studies and suggest that biogenic isoprene may not be as important to the tropospheric ozone chemistry in the Lower Fraser Valley as it is in some southern U.S. cities. ¿ 1998 American Geophysical Union