VIDALIA, Ga. — For years, labor unions and immigrant rights activists have accused large-scale farmers, like those harvesting sweet Vidalia onions here this month, of exploiting Mexican guest workers. Working for hours on end under a punishing sun, the pickers are said to be crowded into squalid camps, driven without a break and even cheated of wages.

But as Congress weighs immigration legislation expected to expand the guest worker program, another group is increasingly crying foul — Americans, mostly black, who live near the farms and say they want the field work but cannot get it because it is going to Mexicans. They contend that they are illegally discouraged from applying for work and treated shabbily by farmers who prefer the foreigners for their malleability.

“They like the Mexicans because they are scared and will do anything they tell them to,” said Sherry Tomason, who worked for seven years in the fields here, then quit. Last month she and other local residents filed a federal lawsuit against a large grower of onions, Stanley Farms, alleging that it mistreated them and paid them less than it paid the Mexicans.

The suit is one of a number of legal actions containing similar complaints against farms, including a large one in Moultrie, Ga., where Americans said they had been fired because of their race and national origin, given less desirable jobs and provided with fewer work opportunities than Mexican guest workers. Under a consent decree with the Equal Employment Opportunity Commission, the farm, Southern Valley, agreed to make certain changes.