The nine-year study, conducted by the U.S. Geological Survey, provides the first nationwide count of intersex fish in American rivers. Overall, 44 percent of the largemouth and smallmouth bass dissected turned out to be intersex, but at some sites 91 percent of the male largemouth bass were affected. Biologist Jo Ellen Hinck's team found intersex males at 34 of 111 sites in eight of nine major river basins, including the Columbia, the Colorado and the Mississippi. The Southeastern U.S. was hit hardest, with intersex bass at every location sampled along the Apalachicola, Savannah and Pee Dee rivers. "Now we need to figure out why," says Hinck, the study's leader.