Fish with both male and female sex tissue have been discovered near Colorado wastewater treatment plants on the South Platte River and Boulder Creek.
Scientists are trying to determine if chemicals that disrupt hormones, such as estrogen, are responsible for the gender-bending phenomenon.
Colorado biologist John Woodling discovered the deformed white suckers about two years ago near two wastewater discharge pipes. Female fish also far outnumber the male fish near the plants.
"This is the first thing that I've seen as a scientist that really scared me," said Woodling, 58, a retired fisheries biologist with the Colorado Division of Wildlife who is now working with the University of Colorado.
Scientists haven't pinpointed which chemicals might be causing the deformities, but endocrine disrupters that mimic or disrupt hormones, especially estrogen, are a leading suspect.
Such chemicals are believed to come from excreted birth-control hormones, natural female hormones, and commonly used detergents that are flushed down toilets and drains.
"We're all concerned about it," said Barbara Biggs, Metro Wastewater Reclamation District's governmental officer. "We don't want to leap to any conclusions yet. There are a lot of estrogen sources in the environment."
Over the last 10 years, scientists have documented the impact of endocrine disrupters on everything from British trout to alligators and polar bears. However, little research has been done on the effects of chronic low-dose estrogen exposure on humans.