The identity of 11 strains ofListeria monocytogenesandL. innocuaisolated from various processed seafood products was confirmed by the polymerase chain reaction (PCR). All species" identities determined by a combination of selective media, -hemolysis, and biochemical fermentation profiles were confirmed by PCR, with one exception. One strain (4-4), previously identified asL. innocuabecause of the lack of hemolysis on blood agar, was shown to beL. monocytogenes. In order to determine whether this isolate was a weakly hemolytic variant and thus potentially virulent, or whether it was truly unable to express listeriolysin (LLO), a number of phenotypic and genetic characterizations were performed. The strain was competent for expression of otherL. monocytogenesvirulence factors and characteristics such as the phosphatidylinositol-specific phospholipase C and the ability to adhere to and invade Caco-2 and J774 cells, but could not grow intracellularly and was avirulent in mice. DNA sequencing of the LLO gene (hly) from strain 4-4 showed that the genetic lesion was a single base-pair deletion, resulting in a frame-shift and subsequent stop codon in the translated open reading frame. Such a mutation is unique from those seen in previously described naturally occurring non-hemolyticL. monocytogenesstrains.