The results of a comprehensive study of the effect of sampling frequency on observations of trends in water quality parameters are reported for a 9-station network in Illinois. The study period covered two discontinuous annual periods from October 1976 - October 1977 and June 1978 - June 1979. Based on an acceptable deviation of 10 percent from the annual daily mean values, optimum sampling frequencies were found to vary from monthly to more often than daily. The average percent deviation due to monthly sampling was found to be acceptable for the following water quality parameters: sodium, chloride, alkalinity, hardness, and total dissolved solids. More frequent sampling seems to be indicated for nitrate, ammonium, and dissolved and total phosphorus. The remaining parameters -- iron, manganese, copper, zinc, and turbidity -- demand more frequent sampling than on a monthly basis to ensure acceptable deviations from long-term means.