We determined mammalian cell invasion and virulence gene (inlA,inlB, and actA) sequences of Listeria monocytogenes strainsbelonging to a molecular subtype (RAPD 9) that often persistsin Danish fish-processing plants. These strains invaded humanplacental trophoblasts less efficiently than other L. monocytogenesstrains, including clinical strains, and they carry a prematurestop codon in inlA. Eight of 15 strains, including the RAPD9 and maternofetal strains, had a 105-nucleotide deletion inactA that did not affect cell-to-cell spread in mouse fibroblasts.The RAPD 9 strains may still be regarded as of low virulencewith respect to human listeriosis.