The behaviour of fecal bacteria was investigated in a turbid estuary which is also a shellfishing area. Sediments are polluted by sewage by the way of settlement, fecal bacteria mixed with estuarine water being able to survive a very long time (T90 are about several days). By measuring the increase of salt tolerance of the strains grown in natural estuarine organic matter, it was demonstrated that Escherichia coli is able to support the salinity of coastal water. Moreover, because light penetration is obstructed by suspended matter, the solar bactericidal effect is very low. A close relationship between T90 and light intensity received by the bacteria is found. These mechanisms can explain the low die-off rate found in turbid areas.