Pseudomonas pseudomallei causes the disease melioidosis, with protean manifestations, protracted clinical course and unpredictable response to antimicrobial treatment. Intracellular location of the organism is suspected to be the cause of these properties. This study was undertaken to examine the intracellular growth of this bacterium. Intracellular growth and survival was assessed at different time intervals, by Gram's stain and electronmicroscopic examination. During the first 5 h, the numbers of P. pseudomallei within phagocytes did not change significantly. By 18-21 h, gram-stained preparations revealed that P. pseudomallei cells completely filled the phagocytes and electronmicroscopy showed evidence of binary fission. During that time the number of cfu of P. pseudomallei growing simultaneously in vitro increased by log10 2-3. The phagocytes remained viable throughout the observation period and retained their capacity to produce an oxidative burst for the first hour of incubation. The ability of P. pseudomallei to survive and multiply in phagocytes shows that it is a facultative intracellular bacterium. This finding is relevant to the selection of antimicrobial regimens, and the management of the disease.