Questar Corp., parent company of Mountain Fuel Supply and other subsidiaries, said 1990 net income increased 13 percent to the highest level in five years as a result of improved earnings from its oil and gas exploration and production and interstate gas transmission businesses.

Questar had net income of $57.52 million or $2.91 per share in 1990 up from $50.75 million or $2.55 per share a year earlier. With the improved earnings, Questar's return on shareholders' equity advanced from 10.9 percent in 1989 to 12.3 percent.Questar approved a 49 cents-per-share quarterly dividend on the company's common stock, payable March 18 to shareholders of record Feb. 22. The company also declared dividends of $2 per share on the 8 percent series and $2.15 on the $8.625 series of preferred stock, payable April 1 to holders of record March 8.

The company said its 1990 performance was enhanced by a 30 percent net income gain in the fourth quarter to $22.26 million or $1.14 per share compared with $17.07 million or $0.84 per share for the year-earlier period.

Earnings from retail natural gas distribution for the quarter improved by $2.99 million over the same period in 1989. The company cited higher deliveries from customer growth, colder weather in the last half of December and reduced operating costs.

"We are very pleased with our continuing progress with earnings and return on equity, which should rank among the best for integrated natural gas companies," said R.D. Cash, chairman.

"We were well on our way to a good earnings year prior to the Middle East situation and the subsequent increase in oil prices. Even in the current unsettled economic and political environment, our core businesses are developing growth opportunities.