This paper presents a summary of the microbiological sampling performed as part of a study entitled "Source Water Assessment: Variability of Pathogen Concentrations" which was funded by American Water Works Association Research Foundation (AWWARF) and the United States Environmental Protection Agency (U.S. EPA). The purpose of this paper is to summarize the overall, spatial and temporal distributions of two waterborne pathogens, Giardia and Cryptosporidium, and related biological indicators including bacteria (fecal and total coliform and Clostridium) and viruses (coliphage). Spatial and temporal patterns of occurrence were also investigated for pathogens, bacteria and viruses. These trends were explored to identify and quantify relevant factors influencing the pattern and occurrence of these microbial contaminants. The overall goal of this project was to use these characteristics to design sampling plans that would effectively define the occurrence of pathogens in a variety of watershed types. However, this paper focuses on summarizing the results for monitoring not on specifying general sampling plans. During this one-year period project, physicochemical and biological samples were collected from five sources in the USA and one in the Province of Ontario, Canada. The six sites were Bull Run Reservoir, Oregon; Allatoona Reservoir, Georgia; Mianus River, Connecticut; Tennessee River, Tennessee; Mississippi River, Iowa; and, Grand River, Ontario. Sample collection began in June 1999 and was completed by June 2000. During this period, physicochemical and biological samples were collected from the influent of each participating water treatment plant. Each utility collected a total of approximately 100 biological samples over the course of the sampling period. Bacteria, protozoa, and viruses including Giardia, Cryptosporidium, Clostridium, and phage were collected following a monitoring plan designed specifically for this project using findings from an earlier AwwaRF project. The monitoring plan used three levels of sampling frequency including weekly sampling, daily sampling (5 sets of 10 daily samples), and 10 event samples. Weekly samples were collected in the beginning of the week. The 5 sets of 10 daily samples were collected daily for ten consecutive working days. The event samples were taken at the discretion of each utility at a time when the operators of the plant suspected conditions that would result in one or more of their monitoring parameters being elevated compared to baseline conditions. Ancillary data comprising both water quality data and hydrology data were also collected. Hydrology data including precipitation and stream flow were measured daily or weekly. Ancillary water quality data included pH, temperature, turbidity, alkalinity and hardness. A simple data management plan was used to collect, store, compile and disseminate all data for this project. Excel spreadsheets were utilized for data input, e-mail for data transmission and a relational database for data compilation. The entire system was web enabled to simplify accessing the available data. The password-protected website was used to disseminate data and analysis to the project team, AwwaRF and Project Committee, and participating utilities. Each group had its own password to access the information system to ensure that only the appropriate parties gained access to their own data. The website was updated on a monthly basis as soon all parties involved in the project processed their data. Includes tables, figures.