Bethesda-based Martin Marietta Corp. yesterday reported first-quarter earnings of $27.8 million, a 26.4 percent increase over the $22 million earned during the same period last year. The aerospace company's profits for the quarter translates to earnings of 70 centsa share compared with 56 cents in the comparable quarter of 1984.

Sales increased 24 percent to $1 billion from $810.3 million in the year-ago period.

The first-quarter results are "the best ever for the corporation on a comparable basis," said Thomas G. Pownall, chairman and chief executive officer of the corporation. Pownall said the profits "largely are a reflection of superior growth in our aerospace business plus a better-than-normal performance by the construction . . . segment of our commercial enterprises."

* General Physics Corp. of Columbia, Md., ended its first quarter with net income of $860,000 (23 cents), 27 percent higher than the $679,000 (18 cents) earned in the comparable 1984 period.

General Physics is a high-technology services company that provides training, operations, engineering and research services for the power and defense industries.

General Physics' net revenue for the quarter was $14.1 million, 48 percent higher than 1984 first-quarter sales of $9.6 million.