According to studies, 57% are Mexican:

http://pewhispanic.org/files/reports/44.pdf

This is alittle old:
The 10.3 million undocumented migrants in March 2004 represent about 29 percent of the almost 36 million foreign-born residents of the United States (Figure 1).

Mexicans make up by far the largest group of undocumented migrants at 5.9 million or 57 percent of the total in the March 2004 estimates.

This share has remained virtually unchanged for the past decade, even as the size of the undocumented population has grown very rapidly. In addition, another 2.5 million undocumented migrants or about 24 percent of the total are from other Latin American countries. About 9 percent are from Asia, 6 percent from Europe and Canada, and 4 percent from the rest of the world (Figure 2).


>>>>
>>>>Mike,
>>>>It always annoys me how people who want something to be done about illegal immigrants are implied to be racist.
>>>
>>>ummmm.....mostly because the illegal immigrants usually being targeted are Hispanic?
>>
>>That's because most of the illegal immigrants ARE Hispanic.
>
>PMFJI, but no, I believe they are Asian.