ICHAEL S. COLOMBO is a debt collector, but not one who goes after small fry — people who refuse to pay for their magazine subscriptions or for credit-card charges that have gotten out of hand. Instead, he pursues stockbrokers and investors who owe money on loans or margin calls. And he does this through the art of firm persuasion.

Mr. Colombo, 40, is in the business of getting people to pay the money they owe brokerage firms and investment banks. For more than 15 years, he did this work for three companies in-house, starting in 1983 at PaineWebber.

A year and a half ago, he opened his own collection shop, the StreetWide Asset Recovery Group in Manhattan, with backing from the Firebrand Financial Group.

Mr. Colombo goes after stockbrokers who leave their companies still owing them money and investors who overextend themselves on margin and can't pay up. Given the state of the economy, he is a very busy man these days. He has contracts with 17 financial institutions, including Berry-Shino Securities, InvestPrivate and J. P. Turner & Company, and he oversees some 300 cases at a time. He receives a percentage of what he recovers.

In the past, many firms either wrote off this debt as a lost cause or assigned its collection to a lawyer, who rarely made it a priority. Mr. Colombo, on the other hand, focuses on these obligations like a pit bull.

"This is what I do," he said. "I know the players. I know the system. I know the collection laws." He estimates that in 19 years, he has recovered more than $45 million, once recouping $1 million in one shot. His biggest single collection so far at StreetWide has been $400,000.

Wall Street is taking notice: this year, he was profiled in Registered Rep, a stockbrokers trade magazine.

Brokers may owe money to their former employers because of loans they took out as new hires. Brokerage houses typically forgive these loans in stages over several years, but if the broker leaves before the contract is up, he owes the balance. Not all are eager to pay up.

Mr. Colombo says brokers sometimes argue that they are off the hook because, after all, the company didn't give them everything it promised.

"The bottom line is — and I tell brokers this all the time — if you had a problem with the firm you went to, God bless you if you want to go to another one," he said. "But you don't get to keep the money. You signed a promissory note and you promised to pay."

If that doesn't work, the two parties go to arbitration before the National Association of Securities Dealers. Mr. Colombo handles that work until the actual hearing, when a lawyer has to take over. He says that only about one-quarter of the cases go that far.

"Mike's excellent at this," said Edward Rose, director for litigation at Investec Ernst & Company in Manhattan, one of StreetWide's clients. "He has the approach of a collection firm, but he has the business background. If you went to a standard collection firm, I don't think they'd understand the nuances of how these debts are created."

Mr. Colombo tells of a recent case in which, a month before a broker was going to leave his midsize firm owing $80,000, he put his house into his wife's name to shield it from any judgment. Instead of first pursuing the broker, Mr. Colombo subpoenaed the wife. That was enough to make her husband deal with his financial obligation, he says.

Some targets are far from Wall Street. About a year ago, Mr. Colombo said, one client gave him a case in Alabama involving a customer who had run up margin debt of $17,000. When the customer refused to pay even after losing in arbitration, Mr. Colombo hired an Alabama lawyer to get a court judgment. The man eventually wrote a check for the amount owed plus interest.

Sometimes, the situation is messier. Mr. Colombo recalls a case while he was collecting in-house. A customer told his broker he wanted to sell short 11,000 shares of eBay.

"But the market went against him and we had to buy the position at a loss of $397,000," he said.

The client never paid up, and the fact that the trade was made in the name of a shell company made it difficult to go after him, Mr. Colombo said. But he was eventually able to win back the debt plus interest and legal fees.

"I had a lawyer write me a three-page memo on why it was uncollectable, but I don't really take that very well," Mr. Colombo said.

Karen M. Cullen hired StreetWide when she was general counsel of Prime Charter Ltd., a New York broker-dealer that was absorbed by Fahnestock Viner Holdings of Toronto. The fee was money well spent, Ms. Cullen said.

"Within a couple of months of hiring Mike, he already collected more than we ever had before," she said.

Mr. Colombo is from Brooklyn, but moved to New Jersey in his teens. He got into the business after leaving college and taking a clerk's job in the new collection department at PaineWebber. He rose to divisional vice president and left in 1996 to create a collections department at Gruntal & Company. In 1998, he went to the GKN Securities Corporation, where, he said, he increased collection revenue from $12,000 to $5 million over two years.

Mr. Colombo said that as his reputation grew, and as firms began asking him for advice on collecting their debt, he decided to pursue an old dream of working for himself. Firebrand Financial, which owns GKN, provided initial funds for StreetWide and owns 80 percent of it.

Does he ever feel sorry for the brokers he goes after?

"I'll be the first guy to say that it's tough for brokers right now," he said. Even so, the biggest concession he will make is to work out a payment plan with those who owe his clients money.

"Of course, the knee-jerk reaction to a collection agency calling up is very guarded, or `Go take a walk, I'm not paying,' yadda yadda yadda," he said. "I say: `Listen, I'm not calling to argue with you. This is what you owe.' Sooner or later they'll get the message."