Foreign travelers flying to the United States outnumbered Americans traveling to foreign destinations by air last year for the first time in aviation history.

Americans traditionally have comprised a majority of those travelers flying between the United States and foreign countries. But for the first time last year, the U.S. citizen share of airline passenger traffic between the United States and other countries fell below 50 percent -- droppng to 48 percent in 1980 from 52 percent in 1979, according to statistics released by the Department of Transportation.

Overall, total passenger traffic between the United States and other countries continued to grow last year, rising almost 6 percent to a total of 39.5 million passengers, but the number of American citizens traveling was down 2 percent while travel by foreigners to the United States was up almost 15 percent.

The U.S. citizen share of the U.S.-international air travel market has been declining steadily over the last few years. In 1977, U.S. citizens still made up 58 percent of the total traffic between the United States and foreign destinations; this dropped to 56 percent in 1978.

A major factor in the change has been the weakness of the dollar relative to other world currencies over the last few years -- although it may be changing now. This made it more expensive for Americans to go abroad but less expensive for travelers from abroad to come to the United States. In fact, words like "bargain" have been bandied about in the United States by foreign visitors who found prices for hotels, food and clothing lower here than in their home countries.

The U.S. government's policy of promoting international aviation competition also is believed to have played a part in increasing the availability of low-fare air travel to the United States.

One factor in the decline in U.S. citizens' travel abroad probably is the economic downturn in the United States, which discouraged some people from traveling abroad.

While the U.S. citizen share of the U.S.-world travel market was declining, so was the share of passenger traffic being carried by the U.S. airlines. U.S. airlines had held onto a steady 51 percent share of the passengers between the United States and other countries during 1977, 1978 and 1979, but their share fell to 49 percent in 1980.

The U.S. airlines carried 2 percent more passengers in 1980 than the year before but the foreign-flag airlines carried more than 10 percent more passengers.

The DOT's air traffic reports are based on statistics collected by the Immigration and Naturalization Service at each U.S. airport that offers international flights.

The DOT's report on 1980 also showed that travel between the United States and the United Kingdom continued to be the leading U.S.-world travel market.

In a related development, the Air Transport Association said yesterday that scheduled domestic and international airline traffic was down 10.6 percent in February from the same month a year ago. Domestic traffic was off 11.9 percent in February while foreign traffic was off 2.9 percent.

ATA spokesman Bill Jackman attributed the decline to weakness in the economy and increased fares sparked by rising jet-fuel prices.