One month after buying Mercantile Bancorp., Firstar Corp. is preparing to merge the St. Louis banking company's asset management unit into its own advisory company.

Firstar Investment Research and Management Co. will absorb Mercantile's advisory arm, Mississippi Valley Advisors. The combined unit would manage $34.6 billion of assets -- $25.1 billion of assets from Firstar and $9.5 billion from Mercantile.

Though operating under a common name, the Firstar and Mercantile operations would keep their full contingent of employees, including portfolio managers and researchers, and their headquarters. The merger is expected to be completed during the first quarter.

The combined unit would be headquartered in Firstar's offices in Milwaukee but would keep open Mercantile's St. Louis advisory center.

Firstar is making limited changes to avoid the kind of post-merger disruptions that can send clients elsewhere, said Mary Ellen Stanek, a Firstar veteran, who will lead the combined operation.

"Our changes will be evolutionary, not revolutionary," she said of an operation that serves corporations, pension funds, trusts, and individual clients.

Firstar is treading softly with changes to avoid alienating key executives at each unit, Ms. Stanek said.

"We have a respect for what each of us has done individually," Ms. Stanek said. "We want to preserve best practices and keep the people who got the funds to where they are today. This is not a cost-cutting exercise."

Analysts said making limited changes is best when melding asset management operations.

"You don't want to risk turning off key people," said Diana Yates, a banking analyst with A.G. Edwards & Sons, St. Louis.

These are the people who, most of the time, have built the businesses and have a following, Ms. Yates said.

Still, Firstar is known for its sales culture, and the company could bring some of that drive to the Mercantile operations, Ms. Yates said.

Ms. Stanek's counterpart at Mercantile, John Blixen, will take a senior position in the combined organization. His title has yet to be determined, Ms. Stanek said.

Firstar and Mercantile will maintain their investment strategies, Ms. Stanek said. Firstar emphasizes a growth style of investing, whereas Mercantile focuses on value and "core" investing, which Ms. Stanek described as a cross between value and growth investing.

In looking at places where product additions are possible, Ms. Stanek said, Firstar has a wealth management operation that could be imported over to the Mercantile side.

She said Mercantile has a wrap account that Firstar is interested in adding to its client offerings.

A decision has not been made about how the companies' mutual funds will be combined.

Firstar's $6.8 billion-asset family of funds carries the Firstar name, and Mercantile's $3.5 billion asset family carries the Mercantile name.

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