Anthem Inc., the nation's second-largest health insurer, has signed up 353,635 people, or 29.1% of Covered California enrollment through Jan. 15. That figure includes 2014 renewals and new enrollees since November.
Covered California declined to release this company data Wednesday, but The Times obtained the information from a public-records request.
San Francisco insurer Blue Shield of California trails Anthem with a 27.2% share, or 331,470 members.
Blue Shield's market share is nearly unchanged from April while Anthem slipped a bit after reaching 30.5%.
Kaiser, the state's largest HMO, made the biggest move up, state records show.
The Oakland-based health plan jumped into third with 260,815 policyholders, a 21.4% share. That's up from 17.3% in April.
That stems partly from Kaiser cutting its rates for 2015 across most of the state after having some of the highest premiums last year.
Some consumers also switched to Kaiser after experiencing problems finding a doctor in the narrow networks offered by Anthem and Blue Shield.
Kaiser runs its own network of hospitals and physician offices.
Meantime, Health Net Inc. lost some ground in Covered California. The Woodland Hills insurer has 204,108 customers as of Jan. 15, or a 16.8% share. That's down from 17.3% in April.
A Health Net spokesman said the company was pleased with its performance and pointed out that some customers left because they became eligible for Medi-Cal, the state's Medicaid program for the poor.
Health Net also dropped a PPO plan popular with people seeking a broader choice of providers.
Together, the exchange's four biggest health plans control 94.5% of the Obamacare market in California. Open enrollment under the Affordable Care Act wraps up Feb. 15.
These enrollment figures will fluctuate since not everyone pays their initial premium after signing up.
Covered California has 1.2 million people who renewed from 2014 or newly enrolled since mid-November. The state wants to hit 1.7 million by Feb. 15.