In given media certain coliform bacteria (Bact. coli mutabile in lactose media and Bact. lactis aerogenes in D-arabinose media) grow only after long lags, a phenomenon variously interpreted in terms of adaptation and of the selection of suitable mutants. The mutational view has been supported on the ground, among others, that at certain stages two colony types can be distinguished in plated samples. Size-distributions of colonies have been examined in detail in the light of this contention, and the conclusion that normal and mutant types can be distinguished in this way is held to be inconclusive.