Microscopic observation of P. pseudomallei revealed that impaired cell division occurred upon the addition of carbenicillin to early exponential phase cultures. Cells were noticeably elongated after 1 h, and continued to increase in length with time. At 6 h filaments were abundant. Spherical bodies, some of which appeared to lyse, were frequently observed along the length of these filaments. Electron microscopy of thin sections confirmed that filament formation proceeded without fission, and constrictions were absent. Replication and segregation of DNA appeared normal; and the density of ribosomes suggested that protein synthesis was not impaired. Two types of inclusion bodies, observed in the nucleoid region of the filaments, were more pronounced compared with untreated cells. It was speculated that one of the inclusions was polyphosphate. The cell walls of the filaments showed decreased electron density. These observations suggest that synthesis of components of the cell wall is impaired by carbenicillin.