Observations are recorded of the formation of secondary colonies of papillae on bacterial colonies (mostly of Bact. lactis aerogenes) grown on solid media containing drugs. The papillae appear to bear no special relation to drug-resistant mutants, since they form in growth on plates containing phenol to which no resistance is developed by the strain of bacteria used. They form only in large, ageing colonies, and when the same total bacterial growth is distributed on the one hand among many small colonies on the plate and on the other hand among a few large ones, it is only in the latter case that the papillae form. Cultures derived from papillae and re-plated sometimes show repeated secondary colony formation and sometimes not.