An investigation is made of the global relationship between seasonal variations of the surface mixed-layer depths derived from monthly climatological hydrographic data and seasonal variations of the surface pigments from monthly satellite ocean color data. At middle and high latitudes of the western North Pacific and the North Atlantic, shallowing of the mixed-layer depth from winter to spring largely explains basin-scale features of the spring bloom of phytoplankton in terms of Sverdrup's critical depth theory. In these areas the spring bloom occurs from middle to high latitudes along with the increase of insolation from winter to spring. In the eastern North Pacific and the Southern Ocean the absence of a spring bloom is difficult to explain using the critical depth theory because Sverdrup's parameters are treated as constants, which in nature vary with physiological and ecological conditions. At northern latitudes the termination of fall bloom corresponds to a deepening in the mixed layer beyond the critical depth. Sverdrup's critical depth theory is found useful as a first step in examining the general pattern of phytoplankton seasonality. ¿ American Geophysical Union 1996