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A large number of bacterial strains from the upper respiratory tract were examined for production of imprints of the colonies on blood agar plates. Moraxella strains regularly grew with “corroding” colonies and most cultures produced spreading zones around the colonies after prolonged incubation. Neisseria (Moraxella, Branhamella) catarrhalis, and N. pharyngis colonies produced marked “corrosion” without spreading zones. Micrococcus mucilaginosus, Staphylococcus aureus and S. epidermidis, and some of the strains of Streptococcus pneumoniae and N. meningitidis produced slight to moderate depressions in the agar, with or without moderate “corrosion”. About 10 per cent of the strains of alpha- and beta-haemolytic streptococci failed to produce imprints on the agar. Of the remaining strains the majority produced slight to moderate depressions with or without “corrosion”. About 12 per cent of the strains produced colonies which were deeply imbedded or infiltrated in the agar. The possibility of an association of these phenomena with other physiological characters, as in some Moraxella species, is discussed.