A multiscale air pollution prediction system has been run semi-operationally during a field experiment in Houston, Texas. This modeling system is based on a nonhydrostatic meterorological model (MM5) that was coupled "online" with the RADM2 chemical mechanism. Biogenic emissions, deposition, tracer transport by convection and turbulence, photolysis, and transport by advection are all treated simulataneously with the meteorology ("online"). During the field experiment, this multiscale meteorology/chemistry model was run in real-time twice a day using four different horizontal resolutions (60km, 15km 5km, and 1.7km) on a massively parallel supercomputer. We will describe the experimental setup and show some preliminary meteorological and chemical results.