In an attempt to elucidate the virulence factors and the pathogenic mechanisms of Providencia alcalifaciens, 36 isolates identified in 1994-1995 in Recife city, Brazil were analysed by PCR to investigate the presence of DNA sequences homologous to virulence genes described in other invasive enterobacteria, as well as their ability to invade HeLa cells, their plasmid profiles and antibiotic resistance patterns. The genetic diversity of the isolates was also analysed by RAPD-PCR. No homologous sequences of virulence genes were observed with any of the P. alcalifaciens isolates studied. Ten isolates had no plasmid and 26 harboured one-to-five plasmids of 147-<6.9 kb. Invasion of HeLa cells was observed in only 10 isolates. No correlation between the plasmid content of the strains, their invasion of HeLa cells or their resistance to antimicrobial drugs could be established. The isolates could be distributed into 10 genotypic groups by RAPD-PCR. Considering the genotypic profile and ability to invade HeLa cells, 7 of the 10 invasive isolates belonged to the same genotypic group. The presence of invasive isolates in the same or a related genotypic group suggests the existence of a clonal lineage responsible for the invasiveness.