Despite their shrinking numbers, Zoroastrians — who follow the Prophet Zarathustra (Zoroaster in Greek) — are divided over whether to accept intermarried families and converts and what defines a Zoroastrian. An effort to create a global organizing body fell apart two years ago after some priests accused the organizers of embracing “fake converts” and diluting traditions.

Image Zoroastrians gathered for an annual fire ceremony last month in Burr Ridge, Ill. A portrait of the Prophet Zarathustra is in the background. Credit... Sally Ryan for The New York Times

“They feel that the religion is not universal and is ethnic in nature, and that it should be kept within the tribe,” said Jehan Bagli, a retired chemist in Toronto who is a priest, or mobed, and president of the North American Mobed Council, which includes about 100 priests. “This is a tendency that to me sometimes appears suicidal. And they are prepared to make that sacrifice.”

In South Africa, the last Zoroastrian priest recently died, and there is no one left to officiate at ceremonies, said Rohinton Rivetna, a Zoroastrian leader in Chicago who, with his wife, Roshan, was a principal mover behind the failed effort to organize a global body. But they have not given up.

“We have to be working together if we are going to survive,” Mr. Rivetna said.

Although the collective picture is bleak, most individual Zoroastrians appear to be thriving. They are well-educated and well-traveled professionals, earning incomes that place them in the middle and upper classes of the countries where they or their families settled after leaving their homelands in Iran and India. About 11,000 Zoroastrians live in the United States, 6,000 in Canada, 5,000 in England, 2,700 in Australia and 2,200 in the Persian Gulf nations, according to the Fezana Journal survey.

This is the second major exodus in Zoroastrian history. In Iran, after Muslims rose to power in the seventh century A.D., historians say the Zoroastrian population was decimated by massacres, persecution and conversions to Islam. Seven boatloads of Zoroastrian refugees fled Iran and landed on the coast of India in 936. Their descendants, known as Parsis, built Mumbai, formerly Bombay, into the world capital of Zoroastrianism.

The Zoroastrian magazine Parsiana publishes charts each month tracking births, deaths and marriages. Leaders fret over the reports from Mumbai, where deaths outnumber births six to one. The intermarriage rate there has risen to about one in three. The picture in North America is more hopeful: about 1.5 births for one death. But the intermarriage rate in North America is now nearly 50 percent.