MIAHUATLÁN, Mexico  During the best of the times, Miguel Salcedo’s son, an illegal immigrant in San Diego, would be sending home hundreds of dollars a month to support his struggling family in Mexico. But at times like these, with the American economy out of whack and his son out of work, Mr. Salcedo finds himself doing what he never imagined he would have to do: wiring pesos north.

Unemployment has hit migrant communities in the United States so hard that a startling new phenomenon has been detected: instead of receiving remittances from relatives in the richest country on earth, some down-and-out Mexican families are scraping together what they can to support their unemployed loved ones in the United States.

“We send something whenever we have a little extra, at least enough so he can eat,” said Mr. Salcedo, who is from a small village here in the rural state of Oaxaca and works odd jobs to support his wife, his two younger sons and, now, his jobless eldest boy in California.

He is not alone. Leonardo Herrera, a rancher from outside Tuxtla Gutiérrez in the southern state of Chiapas, said he recently sold a cow to help raise $1,000 to send to his struggling nephew in northern California.