Clarence McKenzie Lewis Jr., 67, a tax lawyer and community development activist, died Saturday at his Washington home of amyotrophic lateral sclerosis, known as Lou Gehrig's disease.

Mr. Lewis came to Washington in 1947 as a lawyer for the Treasury's Tax Legislative Council. From 1952 to 1961, he was a tax lawyer with the Pittsburgh firm of Kirkpatrick, Pomeroy, Lockhart and Johnson. He then returned here to join the Agency for International Development's Alliance for Progress, where he helped develop taxation plans for Latin American countries.

This work led to his involvement in the late 1960s with privately sponsored development efforts in South America and with inner-city youth organizations in this country.

He was a staff consultant to Youth Organizations United, a national organization of urban youth groups, and helped secure funds for this organization from the Department of Health, Education and Welfare and private sources. He retired in 1975 for reasons of health.

Mr. Lewis was born in New York City. He earned a bachelor's degree in chemistry from Princeton University and graduated from Harvard University Law School in 1936.

During World War II, he was an Army captain in the Chemical Warfare Service during the Italian campaign, in Abruzzi and at Anzio.

From 1955 to 1961, Mr. Lewis was a trustee of the Carnegie Museum and Institute in Pittsburgh.

Survivors include his wife, Alverta Van Dusen Lewis, of the home in Washington; four sons, Clarence III, of Palo Alto, Calif., John, of Alexandria, Edward, of Sante Fe, N.M., and Alan, of Pittsburgh; a sister, Barbara Zinsser, of New York City, and four grandchildren. CAPTION: Picture, CLARENCE M. LEWIS JR.