Primary production was measured at 14 stations covering a wide range of oceanic waters. Measurements were made by both the in situ method (Pi) and the simulated in situ method (Ps) Production v. constant irradiance (P v. I) was also measured. Available photosynthetic irradiance [Eq(350-700) in quanta m^-2 s^-1] was calculated from continuous records of total irradiance and measurements of the percentage submarine transmission of irradiance were made with a quantum meter. Using the P v. I curves and Eq(350-700), primary production at several depths at each station was calculated (P,). Pc was shown to be a precise estimate of Ps at all depths. Pc was also highly correlated with Pi, but both Pc and Pi overestimated Pi at the surface by 40 %. Some experiments at three stations showed that a 2-mm thickness of clear glass placed over surface samples in the measurement of Ps could increase Ps by about 50%. This suggested that U.V. irradiance in surface ocean waters decreased Pi and could explain the overestimates by Pc and Ps. The results showed the need for precise information of spectrai composition of irradiance in studies of primary production but demonstrated the kalidity of Eq(350-700) as an estimate of available photosynthetic irradiance. They also showed that Pc could estimate Pi with a high degree of precision, and that such a calculative method could provide a useful way of continuously monitoring the primary production of large bodies of water for extended periods.