A Canadian volunteer couple, both aged 45 years, came to work in the Madang Province of Papua New Guinea (PNG) in June 1989, the husband as a small business advisor and the wife as a public health nurse.' From arrival, they took their chemoprophylaxis conscientiously, i.e., 300 mg of chloroquine weekly. In January 1991, the husband had an episode of Plasmodium vivax malaria, which was treated with 600 mg of chloroquine daily for 3 days, followed by a course of primaquine, 15 mg daily for 2 weeks. In March 1992, the couple went for a weekend in the bush. After their return, they both developed fever and shaking, one 13 days later and the second on the following morning. Both had Plasmodium fakiparum parasites on blood film.They started a curative dose of chloroquine. As the symptoms worsened, treatment was changed on the third day to quinine, 600 mg twice a day for 3 days, followed by a single dose of Fansidar (sulfadoxine/pyrimethamine) .They left PNG on June 4, 1992 and arrived in Canada via Indonesia on July 1 .They took their last dose of chemoprophylaxis with chloroquine only on July 26,6 weeks after leaving Indonesia. O n August 15, on their return to Edmonton after 5 weeks of holidays at Lake Winnipeg, the husband had fever and shivers. He thought it was flu as the weather was freezing cold. In fact, he was diagnosed with vivax malaria. One week later, his wife developed similar symptoms and was also found to have I? vivax parasites in the blood. They were both successfully treated with a standard dose of chloroquine, followed by 15 mg of primaquine daily for 4 weeks.