Dorothy Swaine Thomas, 77, a demographer and professor emeritus of sociology at the University of Pennsylvania, died Sunday at Suburban Hospital.

Since 1972, she had been professorial lecturer at the Center for Population research of Georgetown University. She lived in Bethesda.

Mrs. Thomas taught at the University of Pennsylanvia from 1948 until retiring in 1970. She was codirector of its Center for Population Studies and had directed a major study resulting in the three-volume "Population, Redistributions and Economic Growth," dealing with human migration in this country between 1870 and 1950.

Born in Baltimore, she was a graduate of Columbia University and received a doctorate from the University of London School of Economics.

Before joining the University of Pennsylvania, Mrs. Thomas held research and academic positions at the University of California at Berkeley Yale University, Columbia Teacher's College, the Carnegie Corp, the Social Science Research Council and the Federal Reserve Bank in New York.

She also had been a technical consultant to the United Nations as well as several U.S. agencies, including the Bureau of the Budget, the Census Bureau and the Agriculture Department.

Mrs. Thomas was the author or co-author of six books and numerous articles. With her husband, the late William I. Thomas, who was a noted sociologist, she prepared the book "The Child in America." He died in the late 1940s.

Mrs. Thomas also was a Fellow of the American Statistical Association and belonged to the American Philosophical Society.

She had taught at the International Institute for Population Studies in India and at Swedish universities on five different occasions.

There areimmediate survivors.