Researchers and students seeking to enter the United States routinely encountered difficulties in the months after the Sept. 11 attacks, but as security checks became faster and more efficient, most could count on receiving a visa or a visa renewal in about two weeks. That appears to no longer be the case.

“I started hearing this back in early November,” said Amy Scott, assistant vice president for federal relations at the Association of American Universities. “We are very concerned that we are losing ground here, that people are missing the opportunities to come to the U.S., to teach, conduct research or just participate in a conference.”

John Marburger, President George W. Bush’s science adviser, said in an interview in the February issue of the magazine Seed that “it should be easier to get into the U.S. as a student,” adding, “We really need to be careful about our openness to the world.”

According to “Beyond ‘Fortress America,’ ” a report in January by the National Academy of Sciences, universities around the world now have the research equipment and infrastructure to compete with their American counterparts. When the United States puts up barriers, the report said, “foreign universities are well positioned to extend competing offers.”

Or as Danielle Guichard-Ashbrook of the Massachusetts Institute of Technology put it: “There are other countries that want these folks. They are the best of the best. They have other options.”

Ms. Guichard-Ashbrook directs the International Students Office at M.I.T. Foreign students eventually make it to campus, she said, although the path may be slow and bumpy and they do not necessarily arrive on time. Problems typically occur if they leave the United States  for family visits or scientific meetings abroad  and then find they need a new visa to return.

She told of one student from the Middle East who agonized when he was called home to the bedside of his dying father for fear he would not be allowed back to his classes. He made the trip, she said, and his return was delayed.