The District, which long has prided itself on taxing low-income residents very little, now taxes them at a higher rate than all but one Washington area jurisdiction, according to a recently released report by the D.C. Department of Finance and Revenue.

In fiscal year 1986, the District was second only to Alexandria in total tax burden on a hypothetical family of four in the lowest income category, earning $20,000 a year, according to the study.

In 1985, the District ranked sixth in taxation of the lowest-income group, which then was defined as having an annual average income of $15,000.

District officials were not available late last week to explain the apparent shift.

The figures for fiscal 1986, which ended Sept. 30, 1986, included taxes on income, sales and use, real property and automobiles.

The new compilations found that the hypothetical family with an income of $20,000 would pay total taxes of $2,208 in Alexandria, $2,011 in the District and $1,928 in Fairfax County, which ranked third.

The bottom-ranked jurisdiction for a low-income family was Charles County, which would have imposed $1,549 in taxes.

Nine jurisdictions were studied by the Department of Finance and Revenue, which releases the statistics every year.

In fiscal year 1985, the District ranked sixth in taxation of the lowest-income family, and fourth in taxation of the next-highest income level.

In fiscal 1986, the District moved into second place in taxation of the second-lowest income category, which that year was defined as a hypothetical family of four with an income of $35,000.

The District also took over first rank among local jurisdictions in 1986 in taxation of the highest income-earners. It taxed a hypothetical four-member family earning $100,000 a total of $11,546.

Alexandria, which had taxed the richest family the most in 1985, was second in 1986 with a total tax burden of $10,718.

Those comparisons can be expected to change during the next few years. The District, Maryland and Virginia all restructured their income-tax systems in 1987 to conform with recent congressional revisions of the federal income-tax code.

Without the restructuring, residents of all three jurisdictions would have faced significant tax increases, because their deductions and credits are based on those allowed by the federal government, many of which were curtailed by the new law.

The changes made by the federal government also reduced tax rates, giving individual taxpayers a net tax reduction.

The Department of Finance and Revenue report said the District's tax changes included rate reductions, an increase in the personal exemption to $885 and a doubling of the standard deduction to $2,000.