WASHINGTON - Fannie Mae said its mortgage portfolio expanded at a 17.4% annual growth rate and its mortgage-backed securities were up 13.6% as the housing market expanded in July.

The agency said its net interest margin increased by 3 basis points, to 116 basis points, in the month.

Libby Snyder, senior vice president for investor relations at Fannie Mae, said the 17.4% annualized increase in the agency's mortgage portfolio to $234.7 billion represented strong growth in the housing market. The portfolio is made up of mortgages the agency holds itself.

In mortgages that the agency securitizes, Ms. Snyder says the growth at a 13.6% annual rate represented an increase to $549.8 billion.

Total business volume in July was $17.2 billion, "the highest level in over a year, reflecting large volumes of fixed-rate mortgages at attractive spreads and a strong homebuying season," the agency said in a release.

Ms. Snyder said serious delinquencies for single-family homes continued at a 17-year low in June, at 0.55%, while multifamily at-risk serious delinquencies increased to 1.02% in June from 0.96% in May.

The 3-basis-point increase in the net margin was a result of a similar increase in the margin on the agency's portfolio to 128 basis points. Snyder said that was not only good news for the agency's stockholders, but indicated the strength of the mortgage industry in general.

Total issuance of mortgage-backed securities was $11.5 billion in July, up form $10.4 billion in June. Mortgage-backed liquidations were $5.4 billion in July, compared with $4.5 billion in June.

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