The planktonic microorganism community of Admiralty Bay, Antarctic Peninsula is being monitored since 2002. This study describes composition, size-structure and biomass of the phytoplankton communities observed during early summer 2010/2011 in the area. Organisms were counted by microscopy and scored as belonging to three size classes: <10 µm, 10 - 80 µm and 80 - 400 µm. In terms of density, this phytoplankton community was dominated by cells < 10 µm (~10⁶ - 10⁷ cells L^-1), confirming the increasing presence of small planktonic autotrophs in West Antarctic Peninsula waters. Organisms > 10 µm dominated in terms of biomass (> 50% of Chlorophyll a) with densities of up to 1.6 x 10⁴ cells L^-1, which is approximately 0.6 times higher than observed for early summer in recent years. Additionally, our results confirm a previously observed phenomenon whereby large-size phytoplankton (> 80 µm), especially pennate diatoms, are replaced by mid-size centric diatoms (10- 80 µm), such as Thalassiosira spp., and small dinoflagellates (e.g. Prorocentrum antarcticum) during warmer early summers.