Morton Hoppenfeld, 56, a senior vice president with The Enterprise Development Co. in Columbia, Md., who was the chief planner of the new town of Columbia, died March 26 at Howard County General Hospital after a heart attack. He lived in Columbia.

Mr. Hoppenfeld was born in New York City. He graduated from the Massachusetts Institute of Technology and earned a master's degree in city planning from the University of California at Berkeley.

He moved to Washington in about 1961 and became chief of urban design with the National Capital Planning Commission. In 1963, he joined James W. Rouse & Co., the builders of Columbia. He directed the planning of the new town and was director of planning and design for the Rouse firm for 12 years.

From 1975 to 1980, Mr. Hoppenfeld was dean of the school of architecture and planning at the University of New Mexico. He then returned here and was a consultant until joining Enterprise Development, another Rouse firm.

Projects he helped to plan and design while with Enterprise include Brown's Arcade in Baltimore, The Waterside marketplace in Norfolk and the Portside marketplace in Toledo.

Mr. Hoppenfeld was a member of the American Institute of Architects and the American Institute of Certified Planners and a fellow of the Urban Land Institute.

His marriage to Lee Hoppenfeld ended in divorce.

Survivors include his wife, Jeanne, and their son, Jacob, both of Columbia; two children by his first marriage, Dan, of Santa Cruz, Calif., and Susan, of Vancouver, B.C.; a sister, Gert Shapiro of Atlanta; and one grandson.