Concentrations of environmental microflora and Listeria monocytogenes were monitored at multiple environmental locations within a seafood-processing facility over the course of 6 months. Concentrations of L. monocytogenes were determined using a most-probable-number (MPN) enrichment procedure. Two floor drains had persistent low concentrations of L. monocytogenes (0.03 to >1,100 MPN/cm2). In comparison, concentrations of the other organisms in the drain were much higher (heterotrophic plate count range of 10(5) to 10(8) CFU/cm2). Concentrations of environmental organisms (heterotrophic aerobic plate counts and counts of pseudomonads, Shewanella spp., Aeromonas hydrophila, and coliforms) were not correlated with concentrations of L. monocytogenes. The 178 confirmed L. monocytogenes isolates from the MPN procedure were further characterized by randomly amplified polymorphic DNA analysis. Sixteen different banding patterns were identified, and nine of the patterns were identified from samples collected on two or more collection dates. From all locations, banding type A was observed in 98 confirmed isolates (55%). Although present, L. monocytogenes was a relatively minor component in the ecosystem of the floor drains in this seafood-processing facility.