An enclosed swab assembly was employed in a survey of 790 samples of nasal mucus collected from calves in 12 herds. It was possible to classify the nasal bacterial flora of calves in three components, basal, supplementary and transient. The potential pathogens, P. multocia, and P.hemolytica, together with micrococci, saprophytic Neisseria and a Streptococcus composed the basal flora and were judged to be widely distributed in the cattle population.A special bacteriological-statistical formula was employed to quantitatively classify the populations of several bacterial species. Enzootic pneumonia of calves was not found to be associated with a characteristic nasal bacterial flora; in contrast, cases of bacterial pneumonia in a well-managed "pool" herd of young bulls were regularly associated with the isolation of P. hemolytica from the nasal secretion. Pasteurella species were capable of dominating the flora for periods of several days at a time in what was judged to be a state of active colonization of the nasal muscosa with shedding of the organisms in large numbers. This state of active colonization did not fore-tell the likelihood of the development of pneumonia in affected calves; periodic suppression of P. hemolytica was observed in closely studied calves.