In two previous papers on the distribution of some nutrients and phytoplankters in the Tamar estuary, anomalies in the vertical profile of phosphate concentration have been demonstrated (Mommaerts, 1969a, b). In the top three metres the water column was stratified so that a dip of no more than 50 cm revealed a PO43- (calculated as phosphorus) variation that would normally be accompanied by a marked change in salinity. On the other hand, salinity determinations revealed no dilution at all. Such unexpected variations occurred two or three times in the layer studied. There was an excellent correlation between the PO43- and NO3- (calculated as nitrogen) concentrations on the one hand and between the silicate concentration and salinity on the other hand. Such a phenomenon could indicate either pollution or biological activity distributed along specific layers. The idea of pollution seemed to be most likely but, to fit the observations, the pollutant would have to match the salinity of the estuary at the precise spot where it enters.