LOS ANGELES, March 4 -- African Americans are being sentenced to prison under California's "three strikes and you're out" law at a rate 13 times that of whites, according to a new study released today, almost exactly two years after the law went into effect.

Although they make up only 7 percent of the general population and 20 percent of all felony arrests, blacks account for 43 percent of those imprisoned under the three strikes law, according to the analysis by the San Francisco-based Center on Juvenile and Criminal Justice. The law mandates sentences of 25 years to life for those convicted of three felonies.

The center also found that 83 percent of those sent to prison under the three strikes law are nonviolent offenders, many of them drug offenders.

"If one were writing a law to deliberately target blacks, one could scarcely have done it more effectively than three strikes,' " said Vincent Schiraldi, the center's executive director.

Popularized first in California and Washington state and then embraced by President Clinton and the House Republicans, the three strikes concept has seized the public imagination over the past two years like no other crime control measure. About half the states have enacted or introduced such laws in their legislatures.

In recent months, criticism of the concept here and elsewhere has tended to focus on the mounting costs associated with it. Courts have become clogged with defendants reluctant to plea bargain to third felonies and states are faced with having to build new prisons to house convicted third offenders for long periods of time. Earlier this month, Los Angeles County asked the state for $168 million to help cover costs of implementing the law that county officials estimated could rise to $300 million next year.

Less attention, however, has been paid to what Schiraldi called "absolutely indefensible racial disparities" in sentencing, which he attributed largely to racial imbalances in the staffs of district attorneys offices.

The center's study, based on data from California's corrections, justice and finance departments, found that five of the state's largest counties -- Los Angeles, Orange, Sacramento, San Bernadino and San Diego -- account for 70 percent of those sentenced under three strikes. However, only 82 of the 1,586 prosecutors in those counties were African American, the report said.

State law leaves it up to district attorneys to decide whether to charge an offender under the three strikes law, which provides that the first two strikes must be serious or violent crimes but that the third can be any felony. Those sent to jail on the third strike must serve a minimum of 85 percent of their sentence. "This is why it's important to have more blacks involved in the policymaking levels," Schiraldi said.

However, Gregg Totten, executive director of the California District Attorneys Association, said the report ignored the fact that a disproportionately high number of victims in third-strike cases against black defendants are also African American. "It is a terribly disingenuous thing to say the system is racist. The fact is, we are prosecuting third-strike cases like any other case, and we are doing it successfully because we're prosecuting guilty people," he said.