The U.S. Geological Survey, in cooperation with the University of Arkansas and the U.S. Department of Agriculture, Agricultural Research Service, collected data in 2004 to determine the occurrence of pharmaceuticals and other organic wastewater constituents, including many constituents of emerging environmental concern, in selected streams in northern Arkansas. Samples were collected in March and April 2004 from 17 sites located upstream and downstream from wastewater- treatment plant effluent discharges on 7 streams in northwestern Arkansas and at 1 stream site in a relatively undeveloped basin in north-central Arkansas. Additional samples were collected at three of the sites in August 2004. The targeted organic wastewater constituents and sample sites were selected because wastewater-treatment plant effluent discharge provides a potential point source of these constituents and analytical techniques have improved to accurately measure small amounts of these constituents in environmental samples. At least 1 of the 108 pharmaceutical or other organic wastewater constituents was detected at all sites in 2004, except at Spavinaw Creek near Maysville, Arkansas. The number of detections generally was greater at sites downstream from municipal wastewater-treatment plant effluent discharges (mean = 14) compared to sites not influenced by wastewatertreatment plants (mean = 3). Overall, 42 of the 108 constituents targeted in the collected water-quality samples were detected. The most frequently detected constituents included caffeine, phenol, para-cresol, and acetyl hexamethyl tetrahydro naphthalene.