We describe a method for simulating photochemical production of O3 in a continental-scale tropospheric model with only six independent chemical variables representing tracers transported in the model. The tracers are two primary hydrocarbon families, CO, NOx, peroxyacylinitrates, and odd oxygen (Ox). A chemical module is developed to compute the production and loss rates of tracers over a model time step of 4 hours, using only information on the tracer concentrations input to the module at the beginning of the time step. Test simulations for summertime conditions at mid-latitudes indicate little loss in accuracy compared to detailed model simulations of chemistry with high time resolution. Minimization of the number of tracers and use of a long time step reduces computer time and storage requirements. In addition, parameterization of the chemical computation is facilitated, allowing further savings in computer time. ¿ American Geophysical Union 1989