1. In a stock homozygous for the dominant Bt-1 allele a mutation occurred from Bt to btm. This new allele is unstable and mutates with a high frequency to Bt. Seeds of btm btm constitution are mosaics of normal and brittle tissue. Germinal mutations are numerous - 7.5% of the seeds on selfed btm btm plants are reverse mutations. The Bt alleles obtained by reverse mutation are stable. The btm allele occasionally mutates to a stable bt allele which is indistinguishable from the old bt allele. While genic modifiers influencing the mutability of the btm allele exist it is evident that this allele is intrinsically unstable, and this case is not similar to the a Dt situation.