Federal officials reported yesterday that students in 4th, 8th and 12th grades had scored modestly higher on an American history test than five years earlier, although more than half of high school seniors still showed poor command of basic facts like the effect of the cotton gin on the slave economy or the causes of the Korean War.

Federal officials said they considered the results encouraging because at each level tested, student performance had improved since the last time the exam was administered, in 2001.

''In U.S. history there were higher scores in 2006 for all three grades,'' said Mark Schneider, commissioner of the National Center for Education Statistics, which administers the test, at a Boston news conference that the Education Department carried by Webcast.

The results were less encouraging on a national civics test, on which only fourth graders made any progress.

The best results in the history test were also in fourth grade, where 70 percent of students attained the basic level of achievement or better.

The test results in the two subjects are likely to be closely studied, because Congress is considering the renewal of President Bush's signature education law, the No Child Left Behind Act.

A number of studies have shown that because No Child Left Behind requires states to administer annual tests in math and reading, and punishes schools where scores in those subjects fail to rise, many schools have reduced time spent on other subjects, including history. In a recent study, Martin West, an education professor at Brown, used federal data to show that during 2003-4, first- and sixth-grade teachers spent 23 fewer minutes a week on history than during 1999-2000.

Given such circumstances, lawmakers and educators are likely to puzzle over why achievement in history has increased. Some suggested that the fourth-grade results were tied to better reading skills.

The tests, known as the National Assessment of Educational Progress, divide achievement levels into basic, proficient and advanced. The 2006 history assessment had the highest percentage of 12th-grade students scoring below basic of any subject tested in 2005 and 2006. And only 1 percent of students at any grade level scored at the advanced level.

The history test was given to a national sample of 29,200 4th-, 8th- and 12th-grade students. Among the results were these:

Some 47 percent of the 12th graders performed at the basic level or above. In 2001, 43 percent were at or above basic.

Sixty-five percent of eighth graders achieved the basic level or better, up from 62 percent six years ago.

Seventy percent of fourth graders attained or exceeded the basic level, compared with 66 percent in 2001. Even this result, however, left 30 percent who, for instance, lacked an ability to identify even the most familiar historic figures or explain the reasons for celebrating national holidays.

''It's heartwarming that the test organizers have found positive things to say, but this report is not anything to break out the Champagne over,'' said Theodore K. Rabb, a professor of history at Princeton who advocates devoting more classroom time to the subject.

The civics exam was given to a national sample of 25,300 4th, 8th and 12th graders. Seventy-three percent of fourth-grade pupils performed at the basic level or better, up from 69 percent in 1998, the last time the civics exam was administered. The scores of 8th and 12th graders showed no change.

''What is most discouraging is that as students grow older and progress through the grades towards adulthood and eligibility to vote, their civic knowledge and dispositions seems to grow weaker,'' said David W. Gordon, superintendent of the Sacramento County School District in California, who is a member of the board that sets policies for the test.

Secretary of Education Margaret Spellings seized on yesterday's results to rebut critics who argue that the federal law has narrowed the curriculum.

''When students know how to read and comprehend,'' Ms. Spellings said, ''they apply these skills to other subjects like history and civics.''

But Kim Kozbial-Hess, a fourth-grade teacher from Toledo, Ohio, who is a member of the assessment board, argued that the test results were not promising enough to justify the federal law's focus on reading and math alone.

''Are we doing well enough in U.S. history that it should continue to be left out of the No Child Left Behind legislation?'' she asked at the Boston news conference.

In Washington, Senators Edward M. Kennedy, Democrat of Massachusetts, and Lamar Alexander, Republican of Tennessee, reintroduced a bill on Wednesday based on the premise that the National Assessment gave history short shrift, testing it every five to seven years instead of every other year as with reading and math. Their legislation would require national history tests every four years, with more students tested.

David McCullough, John Hope Franklin, Douglas Brinkley and dozens of other prominent historians have sent Congress a petition urging the bill's passage.