A Bacillus subtilis mutant which formed dark-brown 'medusa' (M) colonies was obtained. It sporulated at a high frequency, overproduced extracellular protease during sporulation and possessed a high genetic instability with a complex segregation pattern. Segregation was maintained after repeated re-isolation of single M colonies. The major wild-type-like class of segregants (B) was stable, sporulated normally and produced normal amounts of protease. Occasionally segregants were obtained which produced extremely high amounts of protease, sporulated poorly, formed transparent colonies and were either highly unstable (TD) or stable (TDst). Rarely B(D) (stable, normal sporulation and protease overproduction) and W and T (both stable and asporogenous) segregants were produced. The M phenotype was transmitted as a single factor by transformation but not by transduction. The results of transduction experiments suggest the presence of two mutations, ScoC and ScoD. It is proposed that this new segregating system in B. subtilis may result from tandem duplication of part of the bacterial chromosome.