The survey of the agency's scientific staff of 1,400 had a 30% response rate and was conducted jointly by the Union of Concerned Scientists and Public Employees for Environmental Responsibility.
More than half of the biologists and other researchers who responded to the survey said they knew of cases in which commercial interests, including timber, grazing, development and energy companies, had applied political pressure to reverse scientific conclusions deemed harmful to their business.
Bush administration officials, including Craig Manson, an assistant secretary of the Interior who oversees the Fish and Wildlife Service, have been critical of the 1973 Endangered Species Act, contending that its implementation has imposed hardships on developers and others while failing to restore healthy populations of wildlife.
Along with Republican leaders in Congress, the administration is pushing to revamp the act. The president's proposed budget calls for a $3-million reduction in funding of Fish and Wildlife's endangered species programs.
"The pressure to alter scientific reports for political reasons has become pervasive at Fish and Wildlife offices around the country," said Lexi Shultz of the Union of Concerned Scientists.
Mitch Snow, a spokesman for the Fish and Wildlife Service, said the agency had no comment on the survey, except to say "some of the basic premises just aren't so."
The two groups that circulated the survey also made available memos from Fish and Wildlife officials that instructed employees not to respond to the survey, even if they did so on their own time. Snow said that agency employees could not use work time to respond to outside surveys.
Fish and Wildlife scientists in 90 national offices were asked 42 questions and given space to respond in essay form in the mail-in survey sent in November.
One scientist working in the Pacific region, which includes California, wrote: "I have been through the reversal of two listing decisions due to political pressure. Science was ignored — and worse, manipulated, to build a bogus rationale for reversal of these listing decisions."
More than 20% of survey responders reported they had been "directed to inappropriately exclude or alter technical information."
However, 69% said they had never been given such a directive. And, although more than half of the respondents said they had been ordered to alter findings to lessen protection of species, nearly 40% said they had never been required to do so.
Sally Stefferud, a biologist who retired in 2002 after 20 years with the agency, said Wednesday she was not surprised by the survey results, saying she had been ordered to change a finding on a biological opinion.
"Political pressures influence the outcome of almost all the cases," she said. "As a scientist, I would probably say you really can't trust the science coming out of the agency."
A biologist in Alaska wrote in response to the survey: "It is one thing for the department to dismiss our recommendations, it is quite another to be forced (under veiled threat of removal) to say something that is counter to our best professional judgment."
Don Lindburg, head of the office of giant panda conservation at the Zoological Society of San Diego, said it was unrealistic to expect federal scientists to be exempt from politics or pressure.
"I've not stood in the shoes of any of those scientists," he said. "But it is not difficult for me to believe that there are pressures from those who are not happy with conservation objectives, and here I am referring to development interest and others.
"But when it comes to altering data, that is a serious matter. I am really sorry to hear that scientists working for the service feel they have to do that. Changing facts to fit the politics — that is a very unhealthy thing. If I were a scientist in that position I would just refuse to do it."
The Union of Concerned Scientists and the public employee group provided copies of the survey and excerpts from essay-style responses.
One biologist based in California, who responded to the survey, said in an interview with The Times that the Fish and Wildlife Service was not interested in adding any species to the endangered species list.
"For biologists who do endangered species analysis, my experience is that the majority of them are ordered to reverse their conclusions [if they favor listing]. There are other biologists who will do it if you won't," said the biologist, who spoke on condition of anonymity.