Most perplexing, officials said, is that the gap is widest among children of the best-educated parents, where average scores by white students remained relatively steady while those of black students fell back toward the lower achievement found in the late 1970s.
That suggests that what was once considered an urban problem, the consequence of poverty and disadvantage, has become a suburban problem, too, one that may result from lower expectations by teachers, fewer black students taking tougher courses or different attitudes about education among black students.
Such disparity in test performance, which has also been seen from analysis of SAT scores, has become one of the most challenging issues in American education. Evidence of a similar pattern in these tests, the National Assessment of Educational Progress, is more indicative of the situation in U.S. schools because it includes students from all groups, not merely those who are seeking to go to college.
Educators and officials say there is no clear explanation for why progress slipped after the gains of the 1980s in the next decade of remarkable prosperity.
"This is a depressing reversal of the gains made over the past two decades," said Michael T. Nettles, vice chairman of the National Assessment Governing Board, which monitors results of the test. "The alarming part is that since 1992, the gaps have been getting wider."
While there are still gaps between white and Hispanic students, those continued to narrow over the last few years, the test results show.
And while black students are catching up to white pupils in basic math skills, they are slipping further behind whites when asked to do more complicated work, such as fractions, percentages or decimals.
This, officials said Thursday, suggests that efforts by states such as Texas to make sure more students know the basics before they graduate from high school have paid off. The challenge now, they say, is for states to demand even higher levels of achievement.
"Unless we take action to address the gap, then we should expect it's going to continue in this direction," said Mr. Nettles, a professor of education and public policy at the University of Michigan.
"What we've learned from this trend report is that when we take action, we get results."
The tests are given every few years, in different intervals, depending on subject, to samplings of 9-, 13-, and 17-year-olds. While other versions of the national tests have changed to reflect changing curriculums, this test, known as the long-term trend test, has stayed essentially the same over the 30 years it has been administered to about 16,000 students around the country. It is considered the most reliable and unbiased survey of U.S. students' performance.
The overall results revealed some positive signs. Girls, in particular, have made striking gains in math and science, and math scores overall have improved consistently from the 1970s through 1999.
But reading scores remained flat or dropped. And science scores of 17-year-olds were lower in 1999 than they were when the test was first given in 1969.
Education Secretary Richard Riley and other officials emphasized Thursday that black students had made progress since the tests were first given. In almost every subject at every age level, the average scores of blacks were higher than they were in the late 1960s or early 1970s.
The trouble, they said, is a new pattern of slipping scores. For example, in science, the gap in scores for black and white 9-year-olds narrowed to 41 points in 1999 from 57 points in 1970. But that 41 points is up from 37 points in 1996. Among nine indicators â€“ three age groups in three subjects â€“ the gap has widened in seven since 1996.
In some cases, it has slipped back to where it was in the early 1970s. Among 17-year-olds, for instance, the gap in science scores widened to 52 points in 1999 from 47 points in 1996, almost back to the 54-point difference noted in 1969.
The gap has grown most strikingly in ranks of children of better-educated parents.
In reading scores of 17-year-olds in 1971, there was a 44-point difference between white students whose parents had gone beyond high school and their black counterparts.
That narrowed to 27 points in 1990 but grew to 36 points in 1999. While the average white score remained relatively stable, the average black score rose to 279 in 1990 from 261 in 1971 and then slipped back to 268 in 1999.
Michael D. Casserly, executive director of the Council of Great City Schools, a coalition of about 60 school districts, said that cities where the gap had narrowed significantly, such as Houston and Philadelphia, offer preschool to all children, have successful after-school programs, keep class sizes small and require students to take rigorous courses.