Oceanic dimethylsulfide (DMS) is hypothesized to impact cloud formation and solar radiation budget at Earth’s surface. Ultrahigh seawater DMS concentrations, up to hundreds of nM, have been observed in the Southern Ocean, which are commonly attributed to concurrent high phytoplankton biomass. However, phytoplankton biomass cannot fully explain the mechanism leading to those extreme values. Herein, we show the results, including seawater DMS concentrations and other biological and environmental parameters, in the water column collected in austral summer of 2015-2016 at the tip of Antarctic Peninsula. Notably, large-scale ultrahigh seawater DMS (up to 85.2 nM and generally above 40 nM in the upper layer) were observed only in areas with co-existing phytoplankton blooms and abundant microzooplankton (indicated by ciliates, whose abundance and biomass were above 1000 ind L-1 and 2 μg L-1, respectively), suggesting the grazing of abundant microzooplankton cause the ultrahigh seawater DMS during the bloom seasons of Southern Ocean.