Winters in Sweden have become warmer in the 1990s, and as a consequence the timing of ice break-up and the growth and decline of spring phytoplankton has shifted, starting earlier. Even spring temperatures have become warmer, leading to an earlier beginning of the summer phytoplankton growth. The spring-ward shift in phytoplankton population growth has resulted in an extension of the growing season by at least one month. Although mean total phytoplankton biomass from May to October has not increased, the spring and early summer biomass of temperature-sensitive phytoplankton groups, such as cyanobacteria and chlorophytes, has increased in the 1990s. No increase was noted for other phytoplankton groups. Considering that some species of cyanobacteria that commonly occur during a summer bloom, such as Anabaena, Aphanizomenon, and Microcystis, can be toxic, the effect of warmer winters on aquatic ecosystems is potentially far-reaching.