The Marriott Corp. yesterday reported a 36 percent increase in earnings for the third quarter, primarily due to strong performances by the firm's hotel and contract food service operations and use of stringent cost-control programs to offset the recession.

Earnings for the quarter ended Sept. 5 increased from $24.9 million (70 cents a share) to $25.3 million (95 cents). The gain in net income was less than the increase in earnings per share primarily because the company bought about 10 million shares of its common stock since the second quarter last year, the company said in a statement. Sales rose 10 percent from $396.3 million to $436.5 million.

For the first three quarters of the year, Marriott had earnings of $55.1 million ($1.95) up 35 percent from $53.7 million ($1.44). Sales were up 12 percent from $1.1 billion to $1.2 billion.

"As predicted, prevailing economic conditions have affected some of our businesses to a greater extent than in the first half of the year," said Marriott President J.W. Marriott Jr.

For example, while profits for Marriott hotels rose 17 percent and those for contract food services increased 25 percent, restaurant profits were flat and "continued to be hurt by depressed industry conditions, as customer counts remained lower than in 1979," the company said.

Great American theme parks also had flat earnings.