A comparative study was made of the photosynthetic efficiency of seawater samples incubated under either neutral density filters or under blue glass filters which were used to simulate the absorption of ocean water. In the first set of experiments, the two types of filters were matched to transmit the same irradiance of light as measured by a selenium photocell and VG9 green glass filter. In the second set of experiments, water samples were incubated under filters that had been matched with a radiometer, so that they transmitted equal irradiances of visible light. The results of the first set of experiments showed that photosynthesis in seawater samples under the two filters was about the same. In the second set of experiments, the rate of photosynthesis for samples under the blue filters was about 66% higher than that of samples under the neutral density filters. The importance of these findings to primary productivity measurements is discussed.