Two of the fastest-growing companies in the booming mortgage insurance business - Amerin Corp. and MAC Investment Corp. - have been in high gear.

Amerin's first-quarter earnings soared 258% over those of the same period of 1995, to $5.7 million. MAC's earnings grew 28%, to $14.4 million.

The companies provide protection to lenders when borrowers' down payment is less than 20%. The insurers take the first hit if the borrower defaults.

Amerin's net premiums - a key indicator of growth - rose 205%, to $12.1 million.

The Chicago company, which began operations three years ago and went public last year, has already caught Wall Street's eye. Jonathan Gray, an analyst with Sanford Bernstein, calls Amerin "a brilliant business success."

Amerin is now picking up half a percentage point of market share each quarter. After the first quarter, the company was writing 6.5% of all new business in the industry, Mr. Gray said.

Amerin's president, Stuart Brofman, credits the momentum to the company's unique business approach. "We present to lenders something that other insurers don't," Mr. Brofman said.

Amerin shuns its rivals' practice of supplementing insurance offerings with marketing, sales, and other products.

Instead, the company sticks to insurance, wholesaling it to very large banks. Insurers have traditionally offered products on a retail level, making for less efficient distribution, analysts said.

The analysts noted that Amerin's insurance products are about 5 basis points cheaper than its competitors'.

CMAC has built its business by expanding into new territory. The Philadelphia company's entry into the Midwest and Southwest helped first- quarter premiums grow 38%, to $41.9 million.

The geographic expansion has also helped "improve the overall risk dispersion of our book," said CMAC president Frank P. Filipps.

CMAC is now writing 10% of the industry's new business and is positioned seize additional market share, said David Hochstim, an analyst at Bear Stearns & Co.

"We expect growth in CMAC's insurance in force, revenues, and earnings to outpace the industry's averages," Mr. Hochstim said.

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