Septic tank systems are used at 50 percent of roadside rest areas in the United States for onsite disposal of wastewater generated from restrooms and from recreational vehicle waste holding-tank dump stations. The percolation test aids the sizing of septic tank leach fields by determining the percolation value for the soil, an index of the rate of seepage_ of water into the soil. The widely used Public Health Service percolation test procedure defines many aspects of the test, though some details are either discretionary or broadly defined. Comparative percolation tests were conducted to determine whether factors permitted to vary in the Public Health Service procedure could affect test results. Such factors investigated were:(al test hole cross-sectional size;(bl method of excavation of test hole;(cl surface preparation of test hole; and (dl protection of interior surface of test hole. Based on findings of these comparative tests, certain precautions during testing are recommended to eliminate some causes of variation in test results, and a calculation is developed for adjusting raw data from percolation tests for the particular size of the test hole used. An improved percolation test method is proposed.