C. Oliver Islelin Jr., 88, a Loudoun County dairy farmer for more than 50 years who has been president of the National Beagle Club of America, died of cardiac arrest Feb. 4 at his farm in Middleburg.

He was president of the National Beagle Club from 1946 to 1971, and then served as honorary chirman of the organization until the time of his death. He also was the founder and master of the Wolver Hill Beagles pack.

Mr. Iselin was a founding director of the Middleburg National Bank, and had been a director of the Upperville Colt and Horse Show. He had been a member of the vestry of Emmanuel Episcopal Church in Middleburg.

He raised Holstein cattle at his farm, Wolver Hill, in Middleburg.

Mr. Iselin was a native of Newport, R.I. He was a graduate of St. Mark's School in Southboro, Mass., and attended Harvard University.

He served in the Army during World War I.

His wife, the fomrer Elizabeth S. Brown, died in 1977.

Survivors include two daughters, Nancy I. Marburg, of Lutherville, Md., and Barbara I. Sears, of Hamilton, Mass.; a son C. Oliver III, of Middleburg; a half-sister, Hope I. Jones, of Tuscon, Ariz. nine grandchildren, and five great-gradnchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Middleburg Volunteer Fire Department