The Soviet, Union has expelled an American diplomat in retaliation for the secret ouster last month of a Soviet official in Washington who U.S. sources said was caught in espionage activities.

The State Department responded yesterday to the American's explusion by ordering a second Soviet diplomat to leave the United States.

In announcing the second expulsion, the State Department termed as "unacceptable" the ordered ouster from Moscow of Donald Kursch, a first secretary in the economics section of the U.S. embassy for the past 18 months.

"We want to make it absolutely clear that Kursch has done absolutely nothing wrong in the performance of his duties." State Department spokesman John Trattner said. He added that the Soviet diplomat expelled last month was engaged in "flagrantly improper activities . . ."

"Under these circumstances we consider the Soviet action against Kursch to be unacceptable and we are declaring persona non grata an equivalent official of the Soviet embassy in Washington."

U.S. sources indicated that the prompt new U.S. retaliation was caused by what administration officials saw as Moscow's violation of "the rules of the game" - the expulsion of an embassy economics officer for the ouster of a man caught "red handed" trying to obtain sensitive information from U.S. citizens.

These sources made it clear that the U.S. action last month came after American security agencies had obtained evidence of the Soviet diplomat's espionage activities. They refused to identify the Soviet, who had since returned to the Soviet Union, or to give details about the nature of his activities that caused the expulsion.

Kursch was given notice to leave Wednesday night and he is scheduled to depart from the Soviet Union on Friday.

Before going to Moscow, Kursch, 35, served as an economics officer in the U.S. embassy in Budapest and was engaged in vigorous promotion of East-West trade.

While ordering a prompt and tough response to Kursch's expulsion, senior officials were reluctant to discuss the matter, presumably to avoid the possibility of mere retaliatory expulsions and the strain they would put on U.S.-Soviet relations.

During his regular noon briefing. Trattner expressed the belief that the retaliatory moves would not lead to a worsening of relations.

The second Soviet diplomat ordered expelled is a member of the trade mission here. U.S. officials refused to identify him but they said he was given a "reasonable time" to prepare for his departure.

"This is our way of showing displeasure with their retaliation, and we hope this process is not going to go on and on," Trattner said.

The United States and the Soviet Union have followed the principle of reciprocity in treatment of each other's diplomats.

Later year a U.S. serving in Moscow was refused permission to return to his post while on a visit to the United States in turn prevented the return to New York of a Soviet diplomat who was visiting the Soviet Union.