SUMMARY1 Picoplankton community production (0.2–2μm) was investigated over 3 months, June‐September 1991, in Llyn Padarn, a mesotrophic upland lake in north Wales.2 The picoplankton was differentiated into autotrophic algae (<1–3μm) and heterotrophic bacteria (<0.2–1 μm) using differential filtration through a 1 μm pore size Nuclepore filter.3 Efficient separation of these distinct metabolic constituents of picoplankton was obtained. A good correlation (r= 0.81,P3μm) 68.3%.7 Bacteria accounted for about half the picopfankton community production via uptake of EOC. Thus although autotrophic picoplankton were ubiquitous, it is likely that their contribution via primary production to the carbon balance of planktonic environments has be