Three out of four Americans who took a geographic literacy test couldn't locate the Persian Gulf, most had no idea of the U.S. population and the majority were lost when it came to finding Britain, France or Japan.

The test, conducted this spring by the Gallup Organization, ranked Americans sixth in geographic literacy compared with their counterparts in eight other countries. Of young adults in all nine countries, Americans scored last."What was most alarming was not only that young Americans did worse than all other 18- to 24-year-olds, but that they were the only ones in that age group, among nine countries, who did worse than the oldest group tested," National Geographic Society president Gilbert M. Grosvenor said Wednesday.

A separate Gallup survey of a larger sample of Americans found comparable ignorance of geography, but high awareness of environmental issues. Almost all were aware of the global ozone problem and more than two-thirds knew of the acid rain problem in North America.

The international survey, commissioned by the geographic society, tested 10,820 adults in Canada, France, Italy, Japan, Mexico, Sweden, the United Kingdom, the United States and West Germany. Those surveyed were asked to locate 13 selected countries, Central America, the Pacific Ocean and the Persian Gulf on an unmarked world map.

Out of a possible score of 16, the Americans ranked sixth with an average of 8.6, on par with the British (8.5) but significantly better than only the Italians (7.6) and Mexicans (7.4). Top honors went to the Swedes (11.6) and West Germans (11.2), followed by the Japanese (9.7), French (9.3) and Canadians (9.2).

American youth received a score of 6.9, the lowest mark of all youth tested and all other U.S. age groups.

Overall, 75 percent of Americans placed the Persian Gulf in the Red Sea, the Mediterranean, the Black Sea or the Indian Ocean.

Forty-five percent did not know where Central America is located. One third identified Vietnam and less than half the Americans were able to identify the United Kingdom, France, South Africa and Japan.

In a multiple-choice question, 57 percent of Americans tested did not know the current U.S. population. Most participants in Sweden, Japan and Canada chose the correct range from a field of four: 150 to 300 million.

In addition to the international quiz, a more comprehensive battery of 81 questions, including a world map test, was administered to 1,611 Americans.

The results, which Gallup senior study director Jeffrey S. Friedman says can be projected to the U.S. population at large, include:

-Not more than half of adult Americans know that the Sandinistas and Contras are fighting in Nicaragua. Some put the warring groups in Iran, Lebanon or Afghanistan.

-Only 15 percent could identify Mexico City as the world's largest city.

-About one-third could name four of the 16 NATO member countries, another third failed to name any and 16 percent of those questioned conferred membership on the Soviet Union.

-Fifty percent failed to name any members of the Warsaw Pact, 20 percent could name four and more than one in 10 made the U.S. a member.

-Fifty-five percent identified South Africa as the nation where apartheid is an official policy.

While nearly 70 percent of Americans tested agreed that the ability to read a map was a necessity in today's world, more than one third could not pick out the westernmost city on a simple map, the survey indicated.