At the halfway mark, industry giant Anthem Blue Cross is holding on to a slim lead among health insurance providers with a 31% share of California's exchange market.
The Covered California exchange released enrollment figures Tuesday for its 11 health plans from Oct. 1 to the end of December. Open enrollment under the Affordable Care Act runs through March 31.
Anthem, the state's largest for-profit health insurer and a unit of WellPoint Inc., has signed up 155,146 people.
Rival Blue Shield of California isn't far behind with 148,137 exchange enrollees, or 30% market share.
Kaiser Permanente, the state's biggest HMO, lost some ground in December.
Through the first two months of enrollment, Kaiser was a close third with a 24% share. But the Oakland healthcare giant ended last month with 93,415 exchange customers, or 19% share.
Kaiser has some of the highest exchange premiums across the state, and analysts say that could explain the slowdown in enrollment. Initially, Kaiser and other insurers may have been helped by existing customers choosing to stick with their same company in the new marketplace.
About 1 million Californians had their previous health coverage canceled at year-end because it didn't comply fully with the requirements of the healthcare law.
Woodland Hills insurer Health Net Inc. remained in fourth place with 81,488 exchange enrollees, or 16% market share.
State officials said they didn't have data yet on how many people who are signing up were previously insured.
Overall, Covered California said 500,108 people enrolled in health plans through Dec. 31. Two weeks into January, enrollment has jumped to more than 625,000, according to the exchange.
A recent surge of applicants has swamped the state exchange and many of the biggest health plans, such as Anthem. Many customers say they have paid for January coverage, but they haven't been able to use it yet because of problems with their insurance company. Calls seeking help often go unanswered, according to customers.
Insurers have apologized for the poor service amid a crush of new business.
In addition to health plan enrollment, another 584,000 Californians have been deemed eligible for an expansion of Medi-Cal, the state's Medicaid program.
None of these figures include people buying individual health insurance outside the state exchange. Only people who wish to take advantage of federal premium subsidies are required to enroll through the state marketplace.