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