IT HAS NOT BEEN a kind week for presidents in the Yemens. North Yemen's chief of state was assassinated by a bomb in a gift package brought to him by a perhaps-unwitting South Yemeni emissary who was himself killed in the blast, and South Yemen's was executed by erstwhile collegues. The immediate purpose of the assassination in North Yemem is obscure, but it does not seem to have altered the already tense state of affairs between the rival Yemens, and things are expected to go on as they were in San'a. It is on Aden, capital of South Yemen, that most attention is focused, since the change there suggests, if not the presence of a Soviet hand, the possibility of a Soviet gain.

Salim Robaya Ali, the man ousted and killed in South Yemen, was no closet friend of the West. A professed Marxist, he had accepted hundreds of Soviets, East German and Cuban advisers, allowed use of Yemen's port of Aden and its airfields to support communist operations in the Horn of Africa, sent Yemeni troops to help Ethiopia and become a leading sponsor of terrorism, all the while keeping his country desperately poor. Even for him, however, there were limits: He had recently balked at backing Ethiopia's campaign to subdue rebel (and Moslem) Eritrea, and evinced some interest in reaching out to Saudi Arabia and North Yemen and the United States. This was too much, it seems, for his colleagues, whose Marxism apparently is unrelieved by the Moslem and moderate strains that were becoming a shade more evident in Mr. Ali.

On the day of his overthrow, American diplomats had planned to arrive in Aden to try to repair relations, which South Yemen broke off in 1969. The American purpose was to offer South Yemem the beginnings of an alternative to full independence on Moscow. That purpose remains sound. If the new leadership is as resolutely anti-American as first reports make out, it may have no interest in rescheduling the American mission. But there is no point in surrendering uncritically to the notion that events in Aden mark yet another advance of the Red tide. Perhaps the new leaders will prefer to keep South Yemen a Soviet client. Washington should not force that choice upon them.

The United States can let the dust settle secure in the knowledge that it has already bolstered the security of its most important friend in the neighborhood, Saudi Arabia, by the warplane deal. The argument that the Saudis needed the planes to tend to possible threats emanating from their Arab neighbors looks even more plausible now. If one were to pick among possible friends in the Arabian peninsula, one would go with the Saudis.