Move Inc., operator of Realtor.com, has sued a former employee and much-larger competitor Zillow, alleging theft of trade secrets.

Although technically headquartered in San Jose, Move maintains a large technology operation in Westlake Village, where the company was founded. Move said March 17 it had filed suit against Errol Samuelson, former head of its Realtor.com operation, and Zillow.

Filed in King County, Wash. against Seattle-based Zillow, the suit includes claims of breach of contract, breach of fiduciary responsibility and misappropriation of trade secrets. The National Association of Realtors and an affiliate, Realtors Information Network, joined as co-plaintiffs, according to a Move regulatory filing.

“At Move, we take our trade secrets and intellectual property extremely seriously,” Move CEO Steve Berkowitz said in a statement.

Move shares have been down sharply since last summer and competition has intensified between it and the other two major real estate price data companies, Zillow and Trulia. The departure of Samuelson for Zillow on March 5 appears to have precipitated Move’s litigation.

Zillow, with a market capitalization of $3.7 billion, has seen its shares trading in the $90 range, within 10 percent of its 52-week high. Move is a much smaller competitor with a market capitalization just below $500 million. Its shares traded up 2.3 percent to $12.65 on March 18.

In addition to Realtor.com, which is the official website for the National Association of Realtors, Move operates a series of sites devoted to housing, moving and home services. They include Moving.com, SeniorHousingNet and TopProducerSystems, one of several sites for the home selling industry.