Antigenic variation in clones of Paramecium aurelia was studied in 61 combinations of 27 different clones and 24 different antisera. It was found that serum-insensitive variants in serum-sensitive clones could give rise to clones of greater, lesser or the same sensitivity of the original parent clone. One new clone of greater serum-sensitivity than its original parent clone was established by continued selection of serum-sensitive individual paramecia in successive clones. This clone showed from time to time some variation in sensitivity to antiserum for its original parent clone but was found to be usually of higher, and never lower, sensitivity than the parent clone in tests done over a period of one year. Nine serum-insensitive clones were derived by continued selection of insensitive members from 7 different parent clones. Two of these regained full sensitivity to antiserum for their original parent clones in 3 months; 2 remained serum-insensitive for a period of 9 months, but reverted to almost the full sensitivity of the original parent clone toward the close of the 13-month period of examination; 5 remained serum-insensitive throughout the periods of their examination which lasted from 15 to 17 months. A single clone grown in antiserum of less than lethal concentration failed to show much variation in sensitivity to the serum. These results are discussed in relation to morphologic phenomena involving nuclear change in paramecia and in relation to the occurrence of antigenic variants in pure cultures of flagellated bacteria.