The Salmonella typhimurium protein tyrosine phosphatase SptP is a target of the centisome 63 type III protein secretion system. This system is essential for the interaction of these bacteria with host cells. We have shown here by a combination of biochemical and microscopy techniques that S. typhimurium directs the translocation of SptP into cultured epithelial cells. Translocation requires the function of the secreted proteins, SipB, SipC and SipD, as strains carrying mutations in any of the genes encoding these proteins fail to translocate SptP. Microinjection of purified GST–SptP into cultured cells results in the disruption of the actin cytoskeleton and the disappearance of stress fibres. These changes are reversible, as microinjected cells regain the normal appearance of their actin cytoskeleton upon prolonged incubation. Microinjection of the catalytically inactive GST–SptP(C481S) protein results in changes similar to those induced by the wild-type toxin. Furthermore, microinjection of a fusion protein between GST and the first 285 amino acids of SptP also leads to identical disruption of the host cell actin cytoskeleton, indicating that the amino-terminal half of SptP is sufficient to mediate this effect. However, microinjection of a fusion protein between GST and the last 259 amino acids of SptP also disrupted the normal appearance of the cytoskeleton. These results support the hypothesis that SptP is an effector protein arranged in modular domains that may co-operate with each other to exert related functions.