n a study that illustrates how difficult a foe AIDS is, a monkey that was given an experimental vaccine died after the virus changed just one of its genes.

H.I.V., which causes AIDS, is already known to mutate and grow impervious to standard AIDS drugs in at least half of the Americans being treated for the infection.

Now researchers have seen a similar outcome with a vaccine that tries to stop the virus from multiplying. The mutation occurred in one of eight vaccinated rhesus monkeys in a Harvard experiment.

The findings were published in today's issue of the journal Nature.

Scientists described the monkey's death as "more disappointing than surprising."

It does not mean that AIDS vaccines are doomed to fail, they said, but illustrates how the virus will not be easily defeated or even contained anytime soon.

"It is sobering to find that a single- point mutation within the virus can initiate a cascade of events resulting in a clinical vaccine failure and death," said Dan H. Barouch, a clinical fellow at Harvard Medical School and lead author of the study.