Simiyu watershed is one of the relatively mid-size river basins draining into Lake Victoria. The objective of this paper is to investigate the hydrologic response and to build a hydrologic model for the watershed. Hydrologic Simulation Program-Fortran (HSPF) and Watershed Modeling System (WMS) are used to build a long term model for the basin. WMS is used to analyze digital elevation and land use data as a pre-processor whereas HSPF is used to simulate daily stream flows. The initial model run indicated a need for calibration. Some model parameters are, thus manipulated to obtain a better fitting model. The calibrated model is, then, validated on a different data set. Statistical comparisons for calibrated and validated model showed no evidence of a great difference between the simulated and observed data. Hence, the model can be used for future runoff predictions in the basin. On the other hand, continuous enhancement efforts are required to improve the model predictive power.