In 2002, the Bloomington Drosophila Stock Center at Indiana University – one of the world's best known sources of fruit flies – had a consignment destined for overseas returned by the US Postal Service (USPS). The center's codirector, Kevin Cook, learned that the package had been deemed in contravention of USPS international postal regulations. The Bloomington center mails tens of thousands of fly cultures overseas each year, and "this was only the first time a postal inspector had sent back a package," says Cook.

Cook had looked through the USPS's mailing regulations in the past. The domestic rules expressly permitted posting of Drosophila, but the international mailing regulations were less clear. In the past he had interpreted them as allowing international mailing, but now he was less sure. Cook immediately wrote to USPS asking for official clarification and the answer that came back was not the one he was hoping for. "The response was that it was not permitted to mail Drosophila overseas," he says. "We were in a fix."

The USPS regulations are based on the Universal Postal Convention, a document that dates back to the end of the 19th century, which stated that the only live animals that could be mailed were bees, leeches, and silkworms. It turned out that the international mailing of Drosophila had been illegal all along, although the international agreement was being interpreted differently in different nations.

When Cook inquired about how the laws might be changed, he learned that the only way to do such a thing was for the United States or another country to present a proposal to the congress of the Universal Postal Union (UPU), a United Nations agency that acts as a forum for cooperation between 190 national postal services.

Those congresses happen only every four or five years – and the next one wasn't until October 2004, so Cook had time to make his case. In the meantime Bloomington kept sending out flies. "We were in a conundrum," Cook says. "We were in violation of the law, but what were we supposed to do? A lot of scientists around the world rely on us."

In January 2003, he made a written report to USPS urging that the law be changed, explaining why flies are important for biomedical research and that international collaborations are crucial. The USPS received it well, and agreed there was need for change. They forwarded it to the State Department, which would be responsible for presenting it at the UPU meeting in Bucharest.

The State Department official who handled the case then made a strategic decision that Cook thinks may have been crucial in swaying the outcome. He decided that the UPU needed to hear from scientists about the importance of the issue so he, together with Cook and Laurie Thompkins, who coordinates National Institutes of Health grant programs for fruit fly research, carefully sought out three or so politically astute scientists in strategically important countries and asked them to lobby their local UPU representative.

"People have since told me that this was one of the only times that UPU delegates have ever been lobbied about something," Cook says. "It was also probably one of the only times that scientists have got involved in the UPU."

Despite its novelty, or perhaps because of it, the strategy worked. "Whenever the facts were laid out for postal service people in any country, they were very supportive," Cook says. At the UPU meeting, an amendment was voted on and passed, adding members of the Drosophilidae family to the list of officially permitted animals in the post.

This change would have come into force in January 2006 – "which we could have lived with," says Cook – but at a follow-up meeting to the UPU Congress, delegates agreed that the implementation could be moved to May 1. "Again, there was no real problem getting the proposal passed and adopted."

Once the international barrier was down, it was up to national governments to enact the regulations, something the USPS did at the end of May. "Now everything's fine," says Cook. "We can mail flies legally between countries.... It's just such a relief not to have to be concerned about this."