Grazing by microzooplankton on phytoplankton in Manukau Harbour was measured by size‐fractionated dilution experiments at monthly intervals from October 1994 to October 1995. Grazing rates were always highest on the < 5 μn size fraction, the smallest size fraction measured. These rates ranged from 0.3 to 1.3 d−1and were highest in November and March; values did not appear to vary with grazer abundance. Grazing rates on the < 5 μm phytoplankton exceeded growth rate by more than’ a factor of two in June when growth rate was seasonally depressed; but for most of the year the grazing rate averaged about 90% of growth rate. Grazing rates on the 5–22 μm phytoplankton were lower in magnitude, and were a lower percentage of phytoplankton growth rate than those on the < 5 μm size fraction. Grazing on the > 22 μm phytoplankton was measurable only during the February bloom of the large diatomOdontella sinensis. The grazing rate was low, being a small percentage of phytoplankton growth rate in that size fraction. Mathematical simulation of the growth of the < 5 μm phytoplankton in the harbour showed that the observed grazing rates were sufficient to prevent this size fraction from blooming.