The seasonal distribution of autotrophic picoplankton in Lake Constance was investigated over four consecutive years. Cell numbers varied seasonally and vertically over four orders of magnitude (102 to 106 cells ml-1). A horizontal variation by a factor of 3 in abundance and biomass across the different parts of the lake was found during summer stratification. Picoplankton peaks occurred during the phytoplankton spring bloom and in late summer. Low values were characteristic for the clear-water phase in early summer and for autumn-winter. This seasonal pattern differed from that of larger phytoplankton in Lake Constance and from the seasonal distribution of picoplankton known from other lakes and marine environments. Picoplankton was predominated by chroococcoid cyanobacteria of about 0.6 pm3 biovolume. The average cell size increased from winter until early summer. Using HPLC pigment analysis, we identified zeaxanthin and p-carotene as typical picoplankton pigments. Results of the pigment analyses suggest that algae others than picocyanobacteria may be more prominent in the picoplankton size class than derived from routine epifluorescence counting.