A steep decline in the stock price of General Growth Properties Inc. prompted a top executive of the company to sell half of his common shares this week to cover a margin call.

Bob Michaels, General Growth's president and chief operating officer, sold 700,000 shares Tuesday at an average price of $27.13, according to a filing with the Securities and Exchange Commission.

The sale left Mr. Michaels with 690,507 common shares in General Growth, which develops and owns shopping malls.

Mr. Michaels, who joined the Chicago-based company in 1972, serves on the board in addition to his executive post. General Growth owns more than 200 U.S. malls.

General Growth's stock has fallen nearly 50% from its year-ago level primarily because of investors' concerns about the weakening U.S. retail environment and General Growth's $18.4 billion in debt coming due in the next 3½ years.

Few observers doubt that General Growth will manage to refinance its debt.

Even so, investors generally have punished the stocks of debt-heavy companies during the credit crunch.

In 4 p.m. composite trading Friday on the New York Stock Exchange, General Growth's stock closed at $27.48, up 17 cents.

A broker issues a margin call when the value of an investor's securities bought with borrowed money drops below a certain level. In that event, the investor must either deposit more money into the account or sell some of the assets.

"We assume he had modest leverage when the stock was [above] $60, but at the current pricing we believe Mr. Michaels was basically forced to sell," UBS AG analyst Jeffrey Spector wrote in a note to his company's clients Friday.

General Growth spokesman Tim Goebel confirmed that Mr. Michaels's sale was because of a margin call. "This [in] no way reflects a negative opinion on Bob's part whatsoever," Mr. Goebel said.

On Friday, Bernie Freibaum, General Growth's chief financial officer, bought 1,400 shares. He holds more than 7.5 million common shares.

Mr. Michaels's stock sale is among the largest this year by a top executive at a major retail real-estate investment trust.

Write to Kris Hudson at kris.hudson@wsj.com

Hyper City Retail to Focus on Big Box and Multi-Channel Format

Lenders Gain Upper Hand on Freibaum

Post Properties Finds No Takers