She worked at the Red Lobster in Times Square and lived with her husband near Yankee Stadium. Yet one night, returning home from her job, Odine D. discovered that African custom, not American law, held sway over her marriage.

A strange woman was sitting in the living room, and Ms. D.’s husband, a security guard born in Ghana, introduced her as his other wife. Devastated, Ms. D., a Guinean immigrant who insisted that her last name be withheld, said she protested: “I can’t live with the woman in my house — we have only two bedrooms.” Her husband cited Islamic precepts allowing a man to have up to four wives, and told her to get used to it. And she tried to obey.

Polygamy in America, outlawed in every state but rarely prosecuted, has long been associated with Mormon splinter groups out West, not immigrants in New York. But a fatal fire in a row house in the Bronx on March 7 revealed its presence there.

The city’s mourning for the dead — a woman and nine children in two families from Mali — has been followed by a hushed double take at the domestic arrangements described by relatives: Moussa Magassa, the Mali-born American citizen who owned the house and was the father of five children who perished, had two wives in the home, on different floors. Both survived.

No one knows how prevalent polygamy is in New York. Those who practice it have cause to keep it secret: under immigration law, polygamy is grounds for exclusion from the United States. No agency is known to collect data on polygamous unions, which typically take shape over time and under the radar, often with religious ceremonies overseas and a visitor’s visa for the wife, arranged by other relatives. Some men have one wife in the United States and others abroad.

But the Magassas clearly are not an isolated case. Immigration to New York and other American cities has soared from places where polygamy is lawful and widespread, especially from West African countries like Mali, where demographic surveys show that 43 percent of women are in polygamous marriages.

And the picture that emerges from dozens of interviews with African immigrants, officials and scholars of polygamy is of a clandestine practice that probably involves thousands of New Yorkers.

“It’s difficult, but one accepts it because it’s our religion,” said Doussou Traoré, 52, president of an association of Malian women in New York, who married an older man with two other wives who remain in Mali. “Our mothers accepted it. Our grandmothers accepted it. Why not us?”

Other women spoke bitterly of polygamy. They said their participation was dictated by an African culture of female subjugation and linked polygamy to female genital cutting and domestic violence. That view is echoed by most research on plural marriages, including studies of West African immigrants in France, where the government estimates that 120,000 people live in 20,000 polygamous families.

“The woman is in effect the slave of the man,” said a stylish Guinean businesswoman in her 40s who, like many women interviewed in Harlem and the Bronx, spoke on the condition of anonymity. “If you protest, your husband will hit you, and if you call the police, he’s going to divorce you, and the whole community will scorn you.”

“Even me,” she added. “My husband went to find another wife in Africa, and he has the right to do that. They tell you nothing, until one afternoon he says, ‘O.K., your co-wife arrives this evening.’ ”

Don’t-ask-don’t-know policies prevail in many agencies that deal with immigrant families in New York, perhaps because there is no framework for addressing polygamy in a city that prides itself on tolerance of religious, cultural and sexual differences — and on support for human rights and equality.

Islam is often cited as the authority that allows polygamy. But in Africa, the practice is a cultural tradition that crosses religious lines, while some Muslim lands elsewhere sharply restrict it. Many immigrants said, are cramped apartments in the Bronx with many children underfoot, clashes between jealous co-wives and domestic violence. And if the household breaks up, the wives’ legal status is murky at best, with little case law to guide decisions on marital property or benefits.

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