Water contamination is any change in biological properties of water that have a harmful effect on living things. From the beginning of April 2010 till the end of December 2011, One thousand five hundred sixty seven of tap water samples from different parts of Baghdad city were collected, and examined bacteriologic ally by traditional method for detection of pathogenic bacteria. For further detection the molecular study carried out to detected the virulence genes of pathogenic isolates , five isolates of Salmonella spp.gave positive results for the invA gene and prgKgene , three isolates of non-O1 V.cholera were positive for omp Wgene and yielded negative results for the ctxAB gene and the zotgenes but, one of them gave positive result for the tcp gene. Thirteen isolates of Aeromonashydrophila gave positive results for Fla and laf flagellin genes.