A regional chemical transport model (CTM) has been employed to assess the influence of biogenic emissions on ozone (O3) formation in Houston, Texas during an eight-day episode of 24–31 August 2000 in association with the Texas Air Quality Study (TexAQS) 2000. The effects of isoprene and monoterpene emissions on O3 photochemistry in this region were investigated. Isoprene emissions played an important role in O3 formation when the O3 plume occurred in the afternoon over the urban Houston area. When the isoprene emissions were decreased or increased by 50%, the O3 concentration was decreased or increased by 5–25 ppb over the urban Houston area, respectively, but the change in the O3 concentration was less than 5–10 ppb over the industrial Ship Channel. Additional sensitivity studies showed that the surface O3 change resulted primarily from local isoprene emissions, although transport of isoprene from the north of the urban Houston area was found to be nonnegligible in the isoprene budget on several days. The contribution of monoterpene emissions to O3 formation was insignificant due to low emission rate and relatively slow reaction rate.