American television manufacturers said yesterday that if the International Trade Commission revokes a nine-year-old dumping finding against Japanese television makers, the Japanese will flood U.S. markets with imported televisions and ultimately destory the U.S. industry.

American industry representatives told the ITC yesterday the Japanese are producing more television picture tubes than they can sell in their own country. In addition, Taiwan and Korea, which often use Japanese parts, soon will build more televisions of their own, and some European countries are trying to keep the Japanese out. That leaves the United States as Japan's major marketing target, the representatives said.

The ITC, at the request of Sanyo Electric Co. Ltd. and several other Japanese television makers, began hearings Wednesday on whether to change its ruling in one of the largest trade cases in the United States. The Japanese representatives said Wednesday that since the government found in 1971 that the Japanese manufacturers were dumping television sets here, many of the companies have opened assembly plants in the United States and export very few Japanese-built sets here.

Dumping is the sale of goods at prices below their production costs that injure or threaten to injure a domestic industry. A dumping finding requires the assessment of sometimes large duties and requires monitoring of the imported goods. The Japanese contend that these penalties require an enormous amount of paperwork that is unnecessary because they no longer are dumping.

In addition, the Japanese contend they have no reason to ship massive numbers of televisions here because during the last decade they have opened assembly plants here. Seven of 14 television manufacturers in the United States are owned by Japanese firms.