In 2004, the Disinfection Byproducts (DBP) Rule will require small surface water filtration plants to comply with the regulated levels of 0.080 mg/L total trihalomethanes (TTHM) and 0.060 mg/L haloacetic acids (HAA5). Under this regulation, small surface water filtration plants that serve 500 to 10,000 people will be required to take samples quarterly and filtration plants serving less than 500 people will be required to take an annual sample. To determine the full impact of the DBP Rule on Pennsylvania's small surface water filtration plants serving populations of less than 10,000, the Pennsylvania Department of Environmental Protection (department) collected samples at 167 plants across the state for TTHM and 156 plants for HAA5 analysis during the summers of 2000 and 2001. In 2001, 29 filtration plants were retested for TTHM and 23 plants were retested for HAA5. These filtration plants were retested due to their exceedence of 80 percent of the maximum contaminant level for TTHM. Whenever possible, the goal was to collect TTHM and HAA5 sample sets. This means that the TTHM and HAA5 sample sets were collected at the same time and location. In addition, samples were collected at the location of maximum residence time in the distribution. The department's Bureau of Laboratories performed the analysis on these samples during both summers. The results identify TTHM as the main contaminant of concern. Approximately 28 percent of Pennsylvania's small surface water filtration plants exceeded 80 percent of the Maximum Contaminant Level (MCL). These filtration plants may need assistance to comply with the future MCL for TTHM. The levels of TTHM in small surface water filtration plants were similar to those in medium and large filtration plants. The levels of HAA5 in small surface water filtration plants were similar to those of the medium and large filtration plants. Four of the filtration plants out of 156 plants exceeded 80 percent of the MCL and may have trouble complying with the future MCL for HAA5. By contrast, 47 percent of the filtration plants were under the TTHM level of 0.04 milligrams per liter of water (mg/L) and the HAA5 level of 0.03 mg/L. These systems may be eligible for reduced TTHM and HAA5 monitoring as outlined under the DBP Rule. The results of the sampling program provided department staff with water quality information before the regulation takes effect. It also identified filtration plants that could have a potential to exceed the MCL as specified in the DBP Rule. Thus, department staff will have more time to work with the filtration plant staff to resolve problems through operational and/or infrastructure improvements. Without TTHM and HAA5 data beforehand, filtration plants may respond with inappropriate adjustments to treatment such as reducing disinfectant levels and compromise inactivation of microbial pathogens. Furthermore, this is currently the only statewide study on the levels of TTHM and HAA5 in small surface water filtration plants in Pennsylvania and may set the stage on what other states can expect in terms of disinfection byproducts. Includes 5 references, tables, figures.