The construction of a wall is not the answer to illegal immigration into the US, the Mexican president has said.
Mr Fox wants the US to allow illegal Mexican migrants to stay
Immigration was the most pressing challenge in ties with the US but also their greatest opportunity, Vicente Fox said on the second day of his US visit.
It comes amid fierce US debate over an immigration reform bill which could benefit millions of Mexican migrants.
Mr Fox spoke as the US Senate cleared the way for final passage of the bill by voting to limit debate.
"One cannot underestimate the importance of this moment and how complex this issue is for our two nations," he told the Utah legislature.
Mr Fox is visiting the western states of Utah, California and Washington on his four-day trip.
On Tuesday, he gave a speech emphasising trade and energy ties to some 500 business and community leaders in Salt Lake City.
He also touched on the immigration debate, saying Mexico wanted to be part of the solution, not the problem.
"It's not with fences that we are going to solve this problem," he told groups active in the Mexican community in Salt Lake City.
Mr Fox also met the Utah Governor Jon Huntsman, who has said it is important for the Mexican president to hear about the feelings of Western states and their concerns about illegal immigrants burdening health care and prison systems.
The Minuteman project, which opposes illegal immigration, organised a protest outside the Capitol building in Salt Lake City.
"We're overpopulated," Salt Lake City resident Randy Maw said. "There are too many illegals working here."
Mexican officials say the president wants to find out more about new measures to increase security along the border between the two countries.
President Bush recently announced plans to deploy thousands of National Guard troops there and Congress has suggested proposals to build more fencing.
US ILLEGAL IMMIGRANTS
About 11.5m illegal immigrants in the US
Four out of 10 have been in US five years or less
75% were born in Latin America
Most enter via southern US border
California, Texas and Florida host most illegal immigrants
Many work in agriculture, transport and construction
Mr Bush also wants to create a guest-worker programme and provide legal status to some of the millions of illegal immigrants in the US.
However, Mr Fox is not meeting Mr Bush during this visit.
The two leaders used to have what many described as one of the best presidential relationships around.
But these warm ties have cooled in recent months, says the BBC's Emilio San Pedro, amid tension over immigration reform and the border reinforcement plan.
Mr Fox would like the US Congress to vote in legislation allowing millions of illegal Mexican migrants the right to remain and seek citizenship.
Money sent back by Mexican workers in the US is one of the principal sources of revenue for the Mexican economy.
And with a presidential election only weeks away, talking tough on immigration in the US can only help Mr Fox and his party, our correspondent says.