TORONTO — Bank of Montreal, Canada's fourth-largest bank in assets, said it's buying Milwaukee, Wis.-based lender Marshall & Ilsley Corp. in a share swap valued at $4.1 billion, making a long-anticipated move to expand its operations in the U.S.

BMO said Friday that, under the definitive agreement, it's offering 0.1257 of a share for each M&I share. Based on Bank of Montreal's closing stock price in Toronto Thursday, the deal values M&I at $7.75 a share. To keep capital strong after the acquisition, the bank plans to issue additional equity before the deal closes prior to July 31, 2011.

BMO will also repay around US$1.7 billion that M&I received from the government's Troubled Asset Loan Program.

The purchase bolsters BMO's position in the Midwestern states, where it's operated Chicago-based Harris Bank since 1984. The acquisition nearly doubles BMO's deposit base in the U.S., to $92 billion, and more than doubles BMO's branches in the U.S., to 695. It's the latest in a string of U.S. purchases by Canadian banks, which have weathered the financial crisis and recession far better than most of their peers to the south.

M&I, the largest bank in Wisconsin, also has offices in Arizona, Florida, Kansas and Minnesota. The acquisition "transforms our U.S. business," BMO Chief Executive Bill Downe said on a conference call.

The Wisconsin regional lender in October reported its eighth-straight quarterly loss as revenue fell and a drop in its loan-loss reserve wasn't enough to lift it into the black. It has seen its results improve of late, though not as much as some other banks.

Earlier this year, Bank of Montreal bought Rockford, Ill.-based Amcore Bank NA in a Federal Deposit Insurance Corp.-assisted deal that extended its reach in Illinois and Wisconsin. It also said earlier this month when it reported fiscal fourth-quarter results that it would continue making small "tuck-in" acquisitions to build its business.

BMO expects the M&I acquisition to be accretive to earnings in 2013, excluding one-time merger and integration costs.

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