We measured the seasonal variation in phytoplankton biomass and photosynthesis in three size classes at four contrasting sites in Manukau Harbour during 1993–94. Low levels occurred during winter, when the < 5 μm class dominated (43–51% of chlorophylla, 46–58% of photosynthesis). The 5–22 um class dominated during early summer (47–66%, and 52–71%, respectively), whereas the > 22 μm class dominated during a late summer bloom ofOdontella sinensisand other large diatoms, particularly at the inner harbour sites (41–57%, and 43–61%, respectively). Values of the photosynthetic parameterPmBfor the different size classes were similar. Values ofIkfor the > 22 μm class were 22–44% higher on average than those for both the smaller classes. The ratio of photosynthesis to biomass in the euphotic zone—a simple index of potential growth rate—varied seasonally; but at any given time values for all three size classes were similar. Despite this, during summer the biomass of the smaller size classes increased much less than that of the > 22 μm class, implying the smaller cells were lost at a greater rate then.