Dr. Leon E. Truesdell, 98, who retired in 1955 as chief demographer of the Bureau of the Census. died Jan. 12 at the Wisconsin Avenue Nursing Home. He had suffered from a heart ailment.

He came to Washington and began his career with the Census Bureau in 1911 as a statistical editor. He later worked for five years for the Department of Agriculture before returning to the Census Bureau in 1919.

Dr. Truesdell served as assistant chief of the Agriculture Division, then as chief of the population division from 1925 to 1948, and finally, as chief demongrapher until his retirement in 1955. He served as a consultant to the bureau until 1967.

During his years at the Census Bureau, he was instrumental in introducing and developing new population classification systems and sampling methods.

He received an exceptional service award from the Commerce Department in 1950 "for outstanding contribution to census statistics of population and to long term improvement of the professional staff, techniques, and publications of the Bureau of the Census."

He was the author of a number of technical articles, including "Farm Population in the United States," in 1926, works on migrants, and "The Development of Punch Card Tabulation in the Bureau of the Census," published in 1965. He also published a volume of poetry in 1957.

Dr. Truesdell was a native of Rowe, Mass. He earned both bachelor's and master's degrees at Brown University, where he was elected to the Phi Beta Kappa, and a doctoral degree at the Robert Brookings Graduate School in Washington.

He had been president of the Population Association of America. Other professional memberships included the American Statistical Association, the American Economic Association, and the International Population Union.

He had belonged to the Cosmos Club since 1928.

His wife, Constance Cole Truesdell, died in 1957.

Survivors include two daughters, Miriam H., of Washington. and Mrs. Ralph B. DeLano Jr., of Cold Springs, N.Y.; a brother, Raymond C., of Vermont, and two grandchildren.