A major shareholder in Six Flags Inc. has thrown his support behind Redskins owner Daniel M. Snyder's bid to gain control of the nation's second-largest theme park operator.

New York investor Simon Glick, who owns about 9.8 percent of Six Flags, said yesterday in a filing with the Securities and Exchange Commission that after reviewing management's turnaround strategy for the company, he intended to vote in favor of Snyder's proposal to replace two top managers and one director with himself, former ESPN programming whiz Mark Shapiro and local home builder Dwight C. Schar.

For his gambit to succeed, Snyder, Six Flags' largest shareholder, with a stake of about 11.7 percent, needs owners of a majority of company shares to support him. More than 70 percent of Six Flags' shares are controlled by seven investors. In addition to Snyder and Glick, other major investors include Omaha fund manager Wallace Weitz, who controls about 10.6 percent of the shares, and Microsoft founder Bill Gates, who owns about 11.5 percent.

A spokesman for Red Zone LLC, an investment vehicle for Snyder's Six Flags shares, did not return calls yesterday.

Snyder and Shapiro have been crisscrossing the country in recent weeks to meet with Six Flags investors and sell them on their plan to fix the company. Voting on the proposal is expected to be done by the end of the year.

In his filing yesterday, Glick said he believed in Snyder and his team's "ability to enhance the share value" of Six Flags.

Six Flags, which put the company up for sale in August soon after Snyder revealed his plans, responded with a letter to shareholders, urging them not to let Snyder "disrupt our sales process."

Glick is the first major shareholder to disclose his vote in the fight for control of the company. Neither Weitz nor Gates has commented on his intentions.

In the letter to shareholders, Six Flags chief executive Kieran E. Burke said the company had attracted the interest of "a large number" of potential bidders, including theme park operators and private equity firms. Six Flags expects to begin receiving initial offers in the coming weeks and sign an agreement with a buyer by the end of the year "unless the process is disrupted" by Snyder's proposal, which is being considered through a separate process.

Turning the company over to Snyder and his team would be a "risky bet," Burke told Six Flags investors, because the Redskins owner and his partners have no experience in the theme park business. He called their ideas for improving Six Flags "dangerous and potentially destructive" of shareholder value.

Snyder, in material sent to shareholders, said he does not believe a viable buyer will emerge, due in large part to the company's more than $2 billion of debt. He said he would buy additional shares in Six Flags for $6.50 a share if shareholders agree to install his team and if the stock price falls below $6.50. The stock recently has been trading at $7 and above.

Although Six Flags managers were "disappointed" with Glick's decision, they remained "confident" that investors would see that a sale process would "obtain maximum value for all Six Flags stockholders," said spokesman Jeremy Jacobs in an e-mailed statement.

However, Glick, who through an associate declined an interview request, disagreed with the company in his filing yesterday, saying "this is not the correct moment" to put Six Flags up for sale.

Redskins owner Daniel M. Snyder, Six Flags' top shareholder, wants to displace the firm's top executives.