Assets held in exchange-traded funds increased 4.9% in December, according to monthly data released Monday by State Street Global Advisors.

According to SSgA, a unit of State Street Corp., the fund class completed a strong year. As of Dec. 31, 819 exchange-traded funds in the United States managed by 31 fund companies had $776 billion.

ETF assets increased 45% last year as the total number of funds increased 10%. That compares with the Standard & Poor’s 500 Index, which rose 1.9% in December and was up 26.5% for the year.

In 2009, mid-cap ETFs had the largest gain as they added $6.7 billion and assets increased 43.4%.

In December, BlackRock Inc. completed its $13.5 billion acquisition of Barclays to become the industry’s largest ETF provider. BlackRock’s ETF business increased 44.5% to $114.7 billion last year. Already this year, BlackRock has started to introduce new ETFs. On Friday, it launched six municipal bond ETFs, which it said are the first of their kind. They are similar to other muni ETFs except they liquidate at a target date.