WHAT KIND of a police operation is Taiwan running, anyway? The question is ocasioned by the case of Chen Wen-cheng, the Carnegie-Mellon University professor who, while visiting his native Taiwan last month, was found dead one morning outside a library. The previous day and night, Mr. Chen, a permanent resident of the United States, had been interrogated for 13 hours about his supposed pro-Taiwanese nationlist political activities. At first the security police said he had committed suicide. Later the official explanation was amended to include the possibility of an accident. But there is a third possibility: that he died at the hands of the police.

The death of a respected professor from an American university is a sorrowful event. The circumstances add a dark political shadow. The police often show a heavy hand in Taiwan, not least to those of the native Taiwanese majority who may resent the dominance of the island by the mainland Chinese faction, still calling itself the Republic of China, defeated in the Chinese civil war. Mr. Chen, however, was being interrogated about political activities that had ostensibly taken place in the United States, where they are entirely legal and protected. The police in Taiwan could not know about such activities if they had not organized surveillance of the many Taiwanese living in this country, especially at universities. That they have done so has long been known, but, outside of Taiwanese circles, not much attention has been paid to it in recent years.

As it happens, Rep. James Leach (R-Iowa), a former foreign sevice officer who has followed Taiwanese affairs for some time, was in a disinterested position to report that the Chen case is not a one-shot affair. Mr. Leach has detailed a pattern of official surveillance, harassment and intimidation of Taiwanese abroad in which interrogation and imprisonment, and sometimes worse, are the rule. The Taiwan authorities may have felt impelled to step up their vigilance after the United States normalized relations with Peking.

It is a scandal that any part of a pattern of police intrusiveness all too common in Taiwan should be imported to the United States. The State Department has questioned the Taiwanese version of Mr. Chen's death and has asked the FBI for information on Taiwanese surveillance in this country. The matter must be pursued.