Incarceration rates for black Americans dropped sharply from 2000 to 2009, especially for women, while the rate of imprisonment for whites and Hispanics rose over the same decade, according to a report released Wednesday by a prison research and advocacy group in Washington.

The declining rates for blacks represented a significant shift in the racial makeup of the United States’ prisons and suggested that the disparities that have long characterized the prison population may be starting to diminish.

“It certainly marks a shift from what we’ve seen for several decades now,” said Marc Mauer, the executive director of the Sentencing Project, whose report was based on data from the federal Bureau of Justice Statistics, part of the Justice Department. “Normally, these things don’t change very dramatically over a one-decade period.”

The decline in incarceration rates was most striking for black women, dropping 30.7 percent over the ten-year period. In 2000, black women were imprisoned at six times the rate of white women; by 2009, they were 2.8 times more likely to be in prison. For black men, the rate of imprisonment decreased by 9.8 percent; in 2000 they were incarcerated at 7.7 times the rate of white men, a rate that fell to 6.4 times that of white men by 2009.