The presence of coliforms bacteria is one of the most prevalent problems in terms of public health in marine ecosystems over the world. In this study were investigated the physico-chemical properties of seawater and the numbers of total aerobic, total coliform, fecal coliform, E. coli O157:H7 and fecal streptococci in seawater and mussel samples collected from Sinop environs between May and October 2011. The microbiological analysis of seawater samples showed that the difference between total coliform, fecal coliform and fecal streptococci numbers (p<0.05) was significant for each station. However, the difference among total aerobic bacteria numbers for each stations (p>0.05) were not found significant. The difference between whole counting results for mussel samples taken from different sampling sites was not significant (p>0.05), too. Furthermore, the results of the screening assay for the presence of E. coli O157:H7 showed that the strain was not detected in neither seawater nor mussel samples. In conclusion, it was determined that fecal coliform and fecal streptococci counts in the seawater and mussel samples were higher than legal (Turkish Bathing Water and Quality of Fishery Products Regulation) limit values for some stations in Sinop coastal areas.