Immigration investigators have arrested 31 Mexican migrant workers in the past two days in Somerset County, Md., migrant worker camps and charged them with being in the country illegally.

The arrests come at the height of the Eastern Shore tomato harvesting season, according to an Immigration and Naturalization Service official. Another estimated that there are as many as 400 illegal aliens living in 40 to 50 private labor camps in Somerset County fields this season.

The 31 Mexican workers accused of being in the United States illegally were arrested Tuesday and Wednesday after an immigration service investigator received a tip while investigating another case Tuesday, officials said.

Most of those charged were arrested late Tuesday when they were returning to the Somerset's Labor Camp's Mexican barracks, a private camp operated by the Somerset Growers, Inc., a farmer-owned corporation. Officials said that of the 26 arrested at that time, one claimed to have an alien resident card, which serves as proof that an alien has permission to work in the United States.

The five others were arrested yesterday.

Most of them were detained in the Somerset County jail Tuesday before they were allowed to collect their wages and belongings yesterday. They were then taken to the Baltimore immigration service offices for deportation proceedings, officials said.

The 31 have the option of contesting deportation proceedings, the officials said. Should they decline to do so, they will be placed aboard a plane bound for Mexico and deported, they said.