Marine organisms were evaluated as possible biomonitors of heavy metal contamination in marine coastal areas. Concentrations of Cd, Cr, Cu, Pb, and Zn were measured in the green algae Ulva lactuca L., the brown algae Padina pavonica (L.) Thivy, the bivalve mollusc Mytilus galloprovincialis Lamarck, and the two gastropod molluscs Monodonta turbinata Born and Patella cerulea L. collected at six coastal stations in the area of the Gulf of Gaeta (Tyrrhenian Sea, central Italy). The coastal area of the Regional Park of Gianola and Monte di Scauri (a "Protected Sea Park" area) was chosen as a control site. Seawater samples were also collected in each site to assess soluble and total metal concentrations and to gain additional information on both the environmental conditions of the area and possible bioaccumulation patterns. Metal concentrations detected in algae and molluscs did not show significant differences among all stations studied. Moreover, statistical analyses (ANOVA, multiple comparison tests, cluster analysis) showed that the Sea Park station was not significantly different from the others. The hypothesis that the Protected Sea Park would be cleaner than the others must therefore be reconsidered. Data from this study were also compared with those previously obtained from uncontaminated sites in the Sicilian Sea, Italy. The results show clearly differences between these two marine ecosystems. The species examined showed great accumulations of metals, with concentration factors (CFs) higher than 10,000 with respect to the concentrations (soluble fractions) in marine waters. Metal concentrations recorded in this area may be used for background levels for intraspecific comparison within the Tyrrhenian area, a body of water about which information is still very scarce.