Primary production in Pterocladia capillacea was measured by means of oxygen evolution (Winkler's Method) under controlled conditions. The method which utilizes light and dark bottles in studies of productivity in phytoplankton has been adapted for the estimation of primary production in macroscopic benthic algal. Satisfactory results were obtained when 1.0 ml by volume of alga in a 270 ml bottle at 24.0 to 28.0ºC was ilvminated at an incandescent light intensity of 40.0 Klux for a period of 2 hours. This method should be applicable to other algal of similar morphology to P. capillacea.