The serious delinquency rate on Freddie Mac guaranteed single-family loans broke the 3% mark in August, the highest reading ever posted by the mortgage giant.

In its new monthly summary, the GSE said the percentage of loans 90 days or more past due and in foreclosure hit 3.13% during the month, up 18 basis points from July. In August 2008, the government sponsored enterprise had a 1.11% serious delinquency rate.

The huge jump in defaults is driven mainly by Freddie's $172 billion portfolio of guaranteed alt-A loans, which had a 9.44% serious delinquency rate as of June 30.

The Alt-A portfolio includes $144.8 billion of interest-only loans and $11.6 billion of payment option ARMs.

There was good news, though: In August Freddie issued $47.5 billion of MBS, a 7% increase from July.

To date, Freddie has issued $411.2 billion of MBS, compared to $356.8 billion during the same period last year. Freddie reported that its purchases of refinanced loans in August totaled $35.6 billion, an increase of 4.3% from July.

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