Time series measurements of in situ fluorescence, extracted particulate chlorophyll a, primary productivity, extracted adenosine 5'-triphosphate, and fluorescence per cell, as measured by flow cytometry, demonstrate seasonal cycles in fluorescence and chlorophyll concentrations in the North Pacific Subtropical Gyre (22¿ 45'N, 158¿ 00'W). Two opposing cycles are evident. In the upper euphotic zone (0--50 m), chlorophyll a concentrations increase in winter, with a maximum in December, and decrease each summer, with a minimum in June or July. In contrast, chlorophyll a concentrations in the lower euphotic zone (100--175 m) increase in spring, with a maximum in May, and decline in fall, with a minimum in October or November. The winter increase in chlorophyll a concentration in the upper 50 m of the water column appears to be a consequence of photoadaptation in response to decreased average mixed-layer light intensity rather than a change in phytoplankton biomass. In the lower euphotic zone, however, the seasonal cycle in pigment concentration does reflect a change in the rate of primary production and in phytoplankton biomass as a consequence of increased light intensity in summer. These observations have important implications for phytoplankton dynamics in the subtropical oceans and for remote sensing of phytoplankton biomass. ¿ American Geophysical Union 1995