Previously, AIG had used just internal or affiliated auditors for its annual statutory filings.

As in the 2006 study, both the auditor and actuarial markets remained dominated by a handful of firms servicing the larger, group-affiliated insurers, with a significant number of smaller accounting and actuarial firms filling the needs of small independent insurers and those with less standard ownership or profit structures.

Among the larger auditing and actuarial firms, however, there has been some shift in market concentration, in addition to the increasing insurer use of outside actuarial services, Best said.

Based just on the count of insurance company clients, the market share of the "Big Four" accounting firms declined to 74.6 percent as of the 2005 statutory filings, from 78.3 percent in 2004, against a 2.2 percent increase in the universe of audited insurers.

For the top six actuarial firms, overall market share increased slightly to 54.5 percent from 53.1 percent, but that was in a universe that increased 12.2 percent year-to-year. The overall portion of insurers using external actuarial firms for their statutory filings increased to 57.9 percent in 2005, from 53.6 percent in 2004.

Part of the observed market shift may be due to insurers shifting from using the same entity for both auditing and actuarial functions.

The Big Four auditing firms accounted for 98.8 percent of that trade, as measured by the number of insurers using a common source for both actuarial and auditing services, according to Best.