The National Contaminant Biomonitoring Program (NCBP), which was developed and maintained by the U.S. Fish and Wildlife Service (FWS), monitored concentrations of environmental contaminants in freshwater fish over a period of about 20 years. Beginning in the 1960s as a component of the multi-agency National Pesticide Monitoring Program (NPMP) and continuing through the mid-1980s, freshwater fish were periodically collected from a national network of stations for analysis of pesticides, PCBs, and other contaminants. The NPMP/NCBP database of contaminant concentrations is the most extensive of its kind, unique in its breadth and depth with respect to environmental contaminant concentrations in fish. The data and information from the NCBP are sought often by a national and international clientele; especially frequent users are biologists in FWS field offices and USGS Water Science Centers. The fish network was suspended by FWS in 1986 pending development of the Biomonitoring of Environmental Status and Trends (BEST) program. The BEST program and the NPMP/NCBP database were transferred to the National Biological Survey/Service (NBS) in 1993. At that time ongoing studies by NBS Inventory and Monitoring (I and M) and the USGS National Water Quality Assessment Program (NAWQA) generated new data at NCBP sites. The database was subsequently (1996) transferred to USGS. The database derived from the NCBP is unique in its ability to characterize the exposure of free-ranging organisms, including threatened and endangered species, to toxic contaminants.