Although the association between rainfall and mosquito populations is common knowledge, the mathematical relationship between the two variables has been elusive. Indoor resting densities of Anopheles gambiae were determined using the pyrethrum spray collection method and rainfall data for the area was also obtained. An exponential relationship was found between the cumulative mean indoor resting densities and cumulative mean monthly rainfall. An exponential growth model was constructed and it showed a highly significant correlation (R2=0.99) between the modeled and the observed mean indoor resting densities of An. gambiae. The model is temperature sensitive had a habitat carrying capacity of 162 female An. gambiae for study area. The model has the potential to simulate vector populations and malaria transmission rates with given rainfall forecasts. Further studies are required to establish the model parameters for different vector ecosystems.