The magnitude and variability of the maximal quantum yield of photosynthesis were examined in the northwestern Sargasso Sea in April 1985. Maximal quantum yield was calculated from light-limited photosynthetic rates and spectrally-weighted absorption coefficients. The absorption by total particulates collected on a glass fiber filter was partitioned into two components, one associated with living phytoplankton and one associated with other absorbing particles. Two types of maximal quantum yield were calculated: one from the absorption by total particulates and one from the absorption by the phytoplanktonic component alone. Maximal quantum yield calculated from absorption by total particulates was low [0.014 to 0.071 mol C (mol photons)–1] and decreased as the proportion of absorption due to the non phytoplanktonic particles increased. The phytoplanktonic maximal quantum yield was higher [0.033 to 0.102 mol C (mol photons)–1] and varied by a factor of two over a period of two weeks during and following a spring bloom. Use of the phytoplanktonic component of absorption to calculate maximal quantum yield allowed analysis of changes in maximal quantum yield as a function of changes in phytoplankton physiology rather than changes in the amount of absorption by particulate detritus. The pattern of variation in quantum yield was related to nitrogen flux; these data suggest that maximal quantum yield can be predicted from environmental conditions on a regional or seasonal basis.