THE SOVIET UNION took a step backward by expelling Associated Press correspondent George Krimsky, the first American journalist it has ousted since 1970. He was accused of currency violations and espionage; actually he was kicked out for his effective coverage of the Soviet dissident movement. The Soviet expulsion can only deepen American wariness toward a regime that, for all its pretensions of self-esteem, obviously still does not feel enough confidence in itself to permit either independent speaking by a handful of its own citizens or independent reporting by Westerners not under its thumb. Chalk up yet another victory for the prevalence of the KGB mentality in the Kremlin.

The State Department, giving notice, retaliated by expelling a TASS correspondent from Washington. The reprisal apparently puts the United States in the embarrassing position of ignoring, as the Russians ignored, the Helsinki provision by which both states pledged to respect journalists engaged in "legitimate pursuit of their professional activities." But the Department had no alternative if the working conditions and tentures of the remaining American journalists in Moscow were to be at least minimally protected. American news organization are, properly, embarrassed at receiving this sort of official patronage. To reject it, however, is to let the Kremlin restrict and intimidate the American press corps intolerably. It is some consolation that unlike Moscow, Washington did not act against a journalist because of what he wrote.

The ouster of Mr. Krimsky is serious enough. What is more troubling is the suggestion it carries of a Soviet determination to challenge the new administration to a test of wills. It is not hard to imagine voices in Moscow demanding a prompt demonstration that the Kremlin will not tolerate American interest, beyond a certain point, in Soviet dissidents - such as the "concern" the State Department expressed yesterday for detained activist Alexander Ginsburg. In Washington, there are voices demanding that the United States take each Soviet human-rights offense as a political challenge to be met with a firm response. Unless treated with great care on both sides, the situation could get quickly and badly out of hand.