Objective: To phenotypically and molecularly characterize and identify the causal agent of onion (Allium cepa L.) bulb rot in Morelos, Mexico. Methodology: Fluorescent bacteria from onion bulb tissue with symptoms of rot were isolated; the LOPAT test was used to describe them and subsequently they were identified by partial 16S rRNA gene amplification. Pathogenicity in vegetables and plants was evaluated injecting a suspension with 108 CFU m[L.sup.-1] of the pathogen. Results: Phenotypic characterization and 16S rRNA nucleotide sequencing showed 100% identity with Pseudomonas viridiflava as the causal agent of onion bulb rot. The pathogen caused infection in broccoli (Brassica oleracea L.), spring onion (Allium fistulosum L.), purple onion (Allium cepa L.), cauliflower (Brassica oleracea var. botrytis L.), leek (Allium porrum L.), and carrot (Daucus carota L.), as well as in plant species such as jalapeno pepper (Capsicum annuum var. annuum L.), bean (Phaseolus vulgaris L.), and tomato (Solanum lycopersicum L.). Implications: This information is important for agriculture in Mexico. Pseudomonas viridiflava is a bacterial pathogen with high potential to infect new hosts. This is the first report of P. viridiflava causing onion rot in Mexico. Conclusions: Pseudomonas viridiflava is the causal agent of onion bulb rot in Morelos, Mexico. Other vegetables (such as spring onions and leek) can be potential new hosts in Mexico. Keywords: pectinolytic bacteria, 16S rRNA, pathogenicity, vegetables.