Roy O. Larson, 68, a retired senior engineer at the Johns Hopkins Applied Physics Laboratory, where he worked on Navy missile projects, died Tuesday at Holy Cross Hospital in Silver Spring after a stroke.

Mr. Larson joined APL in 1945 as a researcher on various defense projects. He was a member of the team that launched this country's first rocket-assisted vehicles in October 1945. Other projects on which he worked included the design and development of the Terrier, Tartar, and Talos missile systems.

In 1962, Mr. Larson was named assistant supervisor of what became the laboratory's Fleet Systems Department. He participated in Navy fleet exercises involving the test of antiaircraft missiles. He retired in 1977, but continued to work at APL on a consulting basis until his death.

Mr. Larson was a resident of Silver Spring and a member of the St. Luke Lutheran Church.

He was a native of Menomonie, Wis., and attended Stout State College in Wisconsin, earning bachelor's and master's degres in industrial education.

During World War II, he worked on the development of radar, sonar and missiles at Northwestern and Columbia universities.

Survivors include his wife, the former Lorraine O'Neal, of Silver Spring; a son, Neal, of Charlotte Hall, Md.; a daughter, Karen Keenan of Baltimore; three sisters, Marna Wendahl of Appleton, Wis., Eve Krizan of Chicago City, Minn., and Emily Frank of Menomonie; four brothers, Floyd, of Colorado Springs, Oliver, of Sioux Falls, S.D., Lavern, of Menomonie, and Herbert of Zion, Ill., and four grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the building fund of the St. Luke Lutheran Church, Colesville Road and Highland Drive, Silver Spring, Md.