BOSTON - Fidelity Investments' assets in retail accounts declined 8.5% in November, according to monthly data released by the company on Tuesday.

Its assets fell from $550.2 billion in October to $503.4 billion in November, and the size of the average Fidelity account fell from $48,056 to $43,949.

Still, the Boston-based fund company increased its customer rolls, adding 5,000 accounts during the month, for a total of 11.45 million. Fidelity Investments said the asset decline stemmed from market fluctuations.

"What happened with us is consistent with what is going on for most brokerage firms, and that is consistent with what is going on in the market," said James Griffin, a spokesman for Fidelity.

Mr. Griffin said that online trading was also down. Online assets, which make up 42.4% of Fidelity's retail assets under management, fell 4.3%, from $320.9 billion to $307.1 billion.

The number of daily trades slipped 8.4%, from 106,319 to 97,391. Commissionable online trades dropped 10.5%, from 82,857 in October to 74,129 in November, and online mutual fund trading 3.5%, from 24,107 daily trades to 23,263.

Fidelity expects that when results are in for December a holiday slowdown will have dulled trading, Mr. Griffin said, but the company cannot predict what the effect will be on its assets under management. "We are encouraged by the increase in accounts, but we expect fluctuation depending upon what is going on in the marketplace," he said.

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