The nation's two largest steelmakers reported differing first-quarter results yesterday as U.S. Steel Corp. said its decisions to close some plants led to an increase in operating profits and Bethlehem Steel Corp. said slow sales led to a decline in earnings.

U.S. Steel, the No. 1 steel company, reported earnings of $126 million ($1.45 a share). That was down from $137.3 million $1.60) in the comparable 1979 period, when a gain of $90.4 million from accounting changes was included.

David Roderick, the chairman of U.S. Steel, said, "all industry segments were profitable. The decision last year to shut down many marginal facilities produced positive results," he added. Sales rose from $3.1 billion in the 1979 period to $3.2 billion.

Bethlehem Steel said its profits fell 8.1 percent to $54.5 million ($1.25 a share) from $59.2 million ($1.36) in last year's period. Sales rose to $1.9 billion from $1.7 billion.

"The generally reduced level of business in the latter part of 1979 continued into the first quarter of 1980," said Lewis W. Foy, who is retiring as chairman and chief executive of Bethlehem.

Roderick said most of U.S. Steel's income came from businesses other than steel. "Although substantial improvements have been realized in our steel operations, unfairly priced imports continue to adversely affect profitability," he said.

Foy said Bethlehem's raw steel production in the first quarter of 1980 was 4.1 percent below the same period a year ago. Steel shipments were 7.3 percent lower than the first quarter of 1979.

The company's projected capital expenditures this year remain between $550 million and $600 million, Foy said.

U.S. Steel recently filed complaints alleging that producers in seven European countries have sold steel for less than the cost of production in the United States, a practice called dumping.

The International Trade Commission is studying the suits to determine what injury, if any, the company may have suffered due to imports. If U.S. Steel's action is successful, import duties could be levied against offenders.

"While domestic steel shipments declined over one-half million tons in the first two months of 1980, steel mill product imports were 13 percent higher than the comparable 1979 period, amounting to over 16 percent of apparent domestic supply," Roderick said.

"Fair trade in steel has become a myth," he added.

Roderick said steel orders, which wer e on an upward trend through early March, "have softened substantially." He said pressures on the economy, including inflation and high interest rates, "make it difficult to see how an economic downturn can be avoided."

Sperry Group an industrial and scientific combine, said it earned $7.60 a share in the year ended March 31, up from $6.35 the previous year, on a rise in sales to $4.785 billion from $4.179 billion.

Net income rose to $277.09 million from $224.13 million.

Fourth-quarter profits were $87.47 million ($2.32 a share) on revenues of $1.385 million compared with $69.56 million ($1.96) a year earlier on revenues of $1.186 billion.

The order backlog was up 17 percent from a year ago at $3.3 billion.

Foreign exchange translation losses in the final quarter were 36 cents a share against 25 cents a year earlier. For the year, foreign exchange losses were 34 cents a share against 23 cents a year earlier.

Seaboard Coastline Industries Inc. reported an 85 percent gain in earnings to $36.98 million ($2.52 a share) in the first quarter from $19.98 million ($1.37) a year earlier as revenues rose from $507.7 million to $613.12 million.

Several important categories of freight began to show weakness toward the end of the quarter, but coal haulage for electric utilities was up strongly throughout the three months, Chairman Prime F. Osborn said.

Northwest Airlines said it lost $10.9 million in the first quarter as a result of higher fuel costs, lower domestic traffic and a seasonal drop in international travel.

The loss continued a string of dismal financial reports by the nation's major carriers, including UAL Inc., parent of United Airlines, American Airlines Inc., and Pan American World Airways Inc., for the first three months of the year.

In the comparable 1979 period, Northwest said Profits totaled $17.3 million (80 cents a share).

Revenues in the latest quarter rose to $358 million from $279 million in the 1979 period.

Times Mirrow Co. profits dropped to $24.4 million (71 cents a share) in the first quarter from $28.3 million (83 cents) a year ago although revenues increased to $403.3 million from $342.6 million.

The communications company, which published the Los Angeles Times, cited declines in its newsprint and forest products, book publishing and other operations groups' profits and higher interest expenses in announcing the lower earnings.

Increased compensation cost and gambling taxes at Resorts International Inc.'s hotel-casino in Altantic City, N.J., reduced the company's first-quarter earnings from year-ago levels, Resorts said yesterday.

Net income was $14.22 million ($1.12 a share) on revenues of $96.025 million in the last period compared with 1979 first-quarter net income of $18.98 million ($1.51) on $84.29 million in revenues.