ABSTRACT Production of ROS by the leukocyte NADPH oxidase is essential for the destruction of pathogenic bacteria inside phagosomes. The enzyme is a complex of cytosolic and membranous subunits that need to assemble upon activation. Biochemical data suggest that the complex is renewed continuously during activity. Furthermore, it is generally assumed that complex assembly and activity occur in parallel. However, information about the oxidase assembly in individual phagosomes in live cells is scarce. We studied the dynamic behavior of the crucial cytosolic NADPH oxidase component p67phox during phagocytosis by videomicroscopy. p67phox is involved in the regulation of electron flow from NADPH to oxygen, leading to superoxide radical formation inside the phagosome. p67phox-citrine, expressed in myeloid PLB-985 cells, accumulated at the phagosomal membrane during phagocytosis of yeast particles. Using photobleaching techniques (FRAP, FLIP), we demonstrated that p67phox-citrine diffused freely in this phagosomal membrane, but the phagosomal pool of p67phox-citrine did not exchange with the cytosolic pool. This result suggests that once assembled in the NADPH oxidase complex, p67phox is stable in this complex. Furthermore, the time of the presence of p67phox-citrine at the phagosome increased substantially in the presence of complement in the opsonizing serum compared with decomplemented serum. PI(3)P also accumulated around phagosomes for twice as long in the presence of complement. The presence of p67phox-citrine was correlated with the duration of phagosomal ROS production in different opsonization conditions. These data support the critical role of p67phox for ROS production on the level of individual phagosomes.