AN AMERICAN electronics dealer was indicted in Los Angeles last week for illegally exporting switching devices known as krytrons to Israel. A krytron has many industrial and military uses. Among other things, it can be used to build a trigger for a nuclear weapon. Last July three Pakistanis were indicted in Houston for attempting to export krytrons illegally to their country.

In the case of the Pakistanis, the shipment was intercepted by the U.S. Customs before it left the Houston airport. The krytrons for Israel were smuggled out successfully over several years and are now there. Some, the Israeli government has said, have been used for military purposes but only for research with conventional weapons -- specifically, range-finders using lasers. The Israelis have also said that those krytrons still in inventory, presumably unused, will be returned to the United States. How about the others?

Both Israel and Pakistan belong to the short list of countries that either already possess unacknowledged nuclear weapons or are trying to build them. The evidence suggests that Israel has for many years had weapons in the final stages of assembly, capable of being completed very quickly in an emergency. Pakistan is not nearly as close to having weapons but -- despite its denials -- is clearly moving toward them. Both countries have nuclear laboratories and reactors that they refuse to open to international inspection, the basic safeguard by which countries demonstrate their intentions regarding nuclear weapons. Neither has signed the Nuclear Non-Proliferation Treaty.

Both American law and common sense require a high level of vigilance on the part of the United States to maintain stringent control of exports that might prove useful to weapons builders. The international effort to dissuade governments from building these weapons has been on the whole successful over the years, but it has required a great deal of hard work by politicians, diplomats and, as in Los Angeles, policemen and prosecutors. That work is necessary.

Each country that obtains these weapons becomes a reason for others to attempt to get them. India's explosion of its nuclear "device" in 1974 -- India claims the thing was peaceful, not a weapon -- became an incitement to Pakistan to match it. Both Pakistan and Israel are in regions of great tension and longstanding hostilities. The United States has to apply the same rules to both. Having failed to catch the illegal shipments to Israel, unlike the similarly illegal shipment to Pakistan, the United States must now ask to have these devices returned -- all of them. These krytrons are a small element in the struggle to prevent nuclear proliferation, but in this endeavor even the small elements are crucial.