Dr. William D. Curtis, 76, a Washington orthodonist for more than 40 years who was active in professional organizations, died Sunday at Suburban Hospital in Bethesda after a heart attack. He was a resident of Bethesda.

He had maintained a private practice in Washington from September 1938 until retiring in 1979, and had served on the staff of Children's Hospital from 1942 to 1979.

Dr. Curtis was a vice president of the D.C. Dental Society in the late 1940s. He was vice president of Southern Society of Orthodontics from 1954 to 1955, and president of the Washington Dental Club in 1960. He also was a diplomate of the American Board of Orthodontics and a fellow of the American College of Dentistry.

He was a native of Columbia, Mo., and graduated from the University of Missouri there in 1930. At the age of 21 Dr. Curtis was stricken with polio, and lost the use of his legs. In 1931, he married Elizabeth Poulter, a nurse who was caring for him. Mrs. Curtis died in 1978.

In 1935 he graduated from Washington University's dental school in St. Louis, received further training in orthodontics at the Forsyth Dental Infirmary in Boston, then served a residency in orthodontics at Yale University's hospital from 1936 to 1938.

During much of his life, Dr. Curtis spent his summers on Cape Cod and was active in the work of of the Marine Biological Laboratory at Wood's Hole, Mass. Before contracting polio, he had traveled Cape Cod by motorcyle to take part in research projects at the Lab.

Dr. Curtis had been a resident of Bethesda for more than 30 years and was president of the North Washington Lions Club in the mid-1940s.

Survivors include four daughters, Mariwin Curtis of Anna MariaIsland, Fla., Betty Anne Cook of Hastings On Hudson, N.Y., Caroline C. Bloom of West Barnstable, Mass., and Connie Emory of Poolesville; nine grandchildren, and three great-grandchildren.