A genetic probe encoding a virulence gene from Salmonella typhimurium was useful in the detection of Salmonella from feces during an outbreak of salmonellosis at a local dairy. A 3.2-kb BamHI restriction endonuclease fragment of the S. typhimurium virulence plasmid, pStSR100, has been useful as a DNA probe for both detection of Salmonella sp. and characterization of virulence plasmids from numerous field isolates. This virA probe hybridizes to a highly conserved gene carried on the large virulence plasmids of invasive Salmonella isolates. Colony blots prepared from feces directly plated onto MaConkey's agar failed to detect low numbers of Salmonella sp. However, hybridization of the VirA probe to vacuum blots or colony blots prepared from feces in tetrathionate enrichment broth incubated for 16 hours at 37 C was effective for detecting Salmonella sp. and resulted in an 85.9% correlation with culture results. The probe also demonstrated the highly conserved nature (96%) of the virulence gene among S. cholerae-suis isolate plasmids detected using Southern blot analysis.