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The development of the phytoplankton in two years with very distinct weather situations was compared. In 1996, the ice on Jöri Lake III melted in mid June, summer stratification persisted during two months, and ice began to build up again in mid October. In 1997, the ice melted only at the end of July, which strongly influenced the development of the phytoplankton. Stratification persisted during two months and the lake froze up towards the end of October. The average chlorophyll-a concentrations were lower in 1996 than in 1997, which reflects the rather high temperatures and stable weather conditions in 1997 after the late melting of the ice. These observations lead us to suggest that the duration of the ice-free season is less decisive for biomass production than the weather conditions during this period. However, the date and duration of melting of the lake ice strongly influence the development of algal species that are typically observed in early season.