Contemporary phytoplankton and palaeolimnological studies were made of the algal response to acidification and liming in three lakes in Hälsingland, central Sweden (Njupfatet, Sjösjön, Djuptjärn). Surveys and experimental studies of the phytoplankton response to liming were undertaken at Njupfatet, together with an experiment designed to determine the possible role of lake sediments as an inoculum for any new species arriving in the water column. Liming had little quantitative or qualitative effect on the phytoplankton diversity at Njupfatet, but did result in the loss of the dominant contributor to algal biomass, Merismopedia tenuissima. None of the four new species recorded in the lake following liming was hatched from the sediment inoculum experiment. A freeze core from Njupfatet was dated by 210Pb and carbonaceous fly-ash particle profiles were also determined for Njupfatet and Sjösjön, which provided an approximate chronology for the latter lake. Diatom analyses were made of the three lakes and pH and dissolved organic carbon inferred using weighted averaging methodology. Njupfatet had no planktonic diatoms over the last 200–300 years, and was dominated by benthic diatoms, but the diatom-inferred pH suggests that the lake was acidified to pH < 5 prior to liming in 1989. Both Djuptjärn and Sjösjön were dominated by planktonic diatoms, and were acidifying from (c. 1970) prior to liming, Sjösjön from pH 6 to ∼ 5 and Djuptjärn from 6·5 to ∼ 5·5. These pH inferences suggest that at least some lakes in this region are susceptible to atmospheric acidification. The response of the diatoms to liming is discussed, in particular the rapid expansion at Njupfatet, of Cyclotella glomerata a diatom which was also present in Sjösjön for a period prior to acidification, and at other lakes in south-west Sweden. Possible reasons for the expansion of this small centric diatom are discussed.