WILMINGTON, Del. (Reuters)—Warren Buffett and the rest of Berkshire Hathaway Inc.'s board of directors were sued by a shareholder Tuesday over presumed heir-apparent David Sokol's trading in the stock of a company that was later bought by Berkshire.

Mr. Sokol, who resigned from Berkshire Hathaway in March, was also named in the lawsuit. He resigned last month after revealing that he had bought shares of Lubrizol Corp. before pitching the company as a possible acquisition.

The lawsuit filed by Berkshire shareholder Mason Kirby in Delaware's Chancery Court calls for Mr. Sokol to give up any improper gains to Berkshire.

It also calls for Mr. Buffett and other directors, including Vice Chairman Charlie Munger, to compensate Berkshire for the damage they caused to the company's reputation and goodwill.

Mr. Sokol and Mr. Buffett's assistant, Carrie Kizer, who fields Berkshire press inquiries, were not immediately available to comment. Law firm Munger, Tolles & Olson L.L.P., which often represents Berkshire, did not immediately return a call.

Mr. Sokol's trading in Lubrizol raised concerns that he could be charged with insider trading, legal experts have said.

The affair also prompted questions about 80-year-old Mr. Buffett's oversight of the Omaha, Neb.-based conglomerate, which he has led since 1965, and whether controls need to be tightened. Berkshire now owns about 80 companies.

Mr. Sokol has said he left Berkshire to spend more time on family business and philanthropy.

Code of conduct

Berkshire revealed on March 30 that Mr. Sokol had bought more than 96,000 Lubrizol shares in early January. Soon after, he raised the idea of buying the chemical company to Mr. Buffett and made a "passing remark" about owning Lubrizol stock.

The $9 billion acquisition was announced on March 14, boosting the value of Mr. Sokol's personal stake in Lubrizol by $3 million.

Tuesday's lawsuit said Mr. Buffett had violated Berkshire's code of conduct by failing to investigate the holdings of Mr. Sokol, who had chaired the company's MidAmerican Energy unit and oversaw its NetJets Inc. operations.

"Sokol's actions and Buffett's inaction significantly impaired the reputation of Berkshire and constituted breaches of their duty of loyalty to Berkshire and its shareholders," the complaint said.

Mr. Sokol has said he had no inside information and that his Lubrizol purchases were not unethical. Both Messrs. Sokol and Buffett have said they believe Mr. Sokol did nothing unlawful.

Twenty-one of 23 top U.S. investment bankers polled at this month's Reuters Global Mergers and Acquisitions Summit said Mr. Sokol should not have traded in Lubrizol.

Berkshire's Class B shares were up 0.4% at $80.62 in afternoon trading.

The case is Kirby vs. Sokol et al., Delaware Chancery Court, No. 6392.