The historic flight of poverty-stricken whites and blacks from the old South to the great centers of the North has ended, the Census Bureau reported yesterday.

Instead, more poor people are migrating to the South from other parts of the country than are leaving it, according to a study by Dr. Larry H. Long.

For generations, and especially in the three decades since World War II, low-income families flocked from the South to the North, seeking better economic opportunities and causing massive demographic changes in the composition of cities.

Millions of impoverished blacks and poor whites went North. Many subsequently went on welfare, giving rise to the theory that southern poor went North for the express purpose of getting on welfare.

Now, however, despite welfare benefits that remain far higher in most of the North than in the South, the great migration of poor people out of the South has ended. Long said this tends to challenge the theory that availability of welfare benefits induces migration.

From 1967 to 1971, poor people were leaving the South at a net rate of 44,000 a year, Long said, but this trend began to reverse sharply after that.

By the period 1975 to 1977, the South was receiving about 78,000 more poor people from other sections of the country each year than it was losing -- a historic and significant change of patterns.

While the South was gaining more poor than it was losing, according to Long, the reverse was true of the North. By the mid-1970s, "the Northeast and North Central regions showed out-migration of the poor."

The main factor in these switches, Long said, appears to be that low-income persons are staying in the South, "probably because of a steady growth of jobs that lift people above the poverty level." The flood of southern poor who once made their way to the North has fallen to a trickle, and the result is a net in-migration of poor to the South.

Until the 1960s, the South was losing all types of people to other regions: poor, nonpoor, white and black. In the 1960s, according to Long, the overall numbers began to shift, largely because nonpoor whites ceased leaving and others in the same category began arriving.

But it was not until the 1970s that the South began "to have net in-migration of blacks and persons below the poverty level" as well as nonpoor whites.

Long said the latest developments mark "a turning point in southern migration: the South now has a net inmigration of practically every demographic group -- white, poor and black."

Reviewing the history of total population shifts to and from the South, Long said the South throughout the 19th century and well into the 20th had most of the characteristics of a depressed area: it was poor, agrarian and with low economic opportunity.

Usually, people of all types migrate from such regions to areas where they can find work. However, the influx of European immigrants into the North and then the Depression of the 1930s reduced economic opportunities for southeners who otherwise would have gone North.

However, starting with World War II these obstacles disappeared and there was a "massive" migration of all people, especially the poor, from the South to other sections. The South, in effect, exported its poor to the rest of the country and "definitely contributed to poverty in other regions," according to Long.

Now, Sun Belt prosperity and opportunity have changed all this.