Geiger and liquid scintillation activities of C¹⁴ labelled phytoplankton filters obtained from a tropical coastal station were compared. A decrease in the percent efficiency of GM/Sci with increasing primary production was observed and this is related to an increase in the thickness of the sample of which is caused either by the productive algal blooms or by other particulate matter. If primary production is not to be underestimated in coastal turbid waters, it will be necessary to determine the absolute activities of the samples.