Gamma A is a widespread non-cyanobacterial diazotroph that plays a crucial role in marine ecosystems, but its controlling factors are still largely unknown. This study, for the  first time, quantified microzooplankton grazing on Gamma A and revealed the significance of grazing pressure on Gamma A distribution around the Kuroshio region. It highlights the importance of top-down controls on Gamma A abundance and the associated nitrogen cycle.