The mean annual production by phytoplankton in St. Margaret's Bay, Nova Scotia, is estimated to be 190 gC/m2 yr. on the basis of 120 in situ14C experiments made during the period June 1966 to June 1969. A method is described for the conversion of the results of short-term 14C experiments into daily and annual rates. No significant difference was found between the production at a station inside the bay and a station outside the bay. Annual variations in primary production were marked; much of the variation could be attributed to the influence of weather on the hydrographic regime of the bay.