The growth of two strains ofPseudomonas aeruginosa in stirred batch cultures was monitored by optical density, DNA concentration, and acridine orange direct cell count measurements. Growth of adherent bacteria in pure culture was also observed on suspended glass discs by light and scanning electron microscopy. Strain MUCOID produced significant numbers of filamentous cells in broth culture and in the adherent population, while strain PAO 381 did not produce elongated cells. Filamentous growth of MUCOID could be prevented by the addition of 5 × 10−2 M Mg2+. However, the addition of 0.66 mM EDTA caused an increased proportion of the population (>50%) of MUCOID cells to become filamentous in broth culture. The results are discussed and related to theories regarding bacterial plasticity, and filamentation of normally bacillary cells.