The NASA Global Ocean Data Set of plant pigment concentrations in the upper euphotic zone is evaluated for discerning geographical and temporal patterns of seasonality in the open sea. Monthly medians of pigment concentrations for all available years are generated for fields of approximately 77,000 km2. For the climatological year, highest and lowest medians, month of occurrence of the highest median, ratio of highest to lowest medians, and absolute range between the highest and lowest medians are mapped ocean-wide between 62.5 ¿N and 62.5 ¿S. Seasonal cycles are depicted for 48 sites. In much of the offshore ocean, seasonality of pigment is inferred to be driven almost equally by the interaction of the abiotic environment with phytoplankton physiology and the loss of cells from grazing. Special emphasis among natural domains or provinces is given to the Subantarctic water ring, with no seasonality in its low chlorophyll concentrations in spite of strong environmental forcing, and the narrow Transition Zones, a few degrees of latitude on the equatorial sides of the Subtropical Convergences of the southern hemisphere and their homologs in the northern hemisphere, which have late winter blooms caused by nutrient injection into the upper layers. ¿ American Geophysical Union 1994