The Holocene development of Pieni Majaslampi, a small hilltop lake which is exceptionally old for the conditions prevailing in southern Finland, is studied here by sedimentological and biostratigraphical methods. The lake is naturally acidic, with a pH of 4.5, and has been so since at least 8500 C14 BP, as determined from inferred-pH techniques based on contemporary diatom-pH relationships. The lake was found to have been subjected to extremely pronounced acidification during its early developmental stages, the pH having dropped from 5.8 to 4.7 between c. 10,000 and 8500 BP. Predicted pH values ranged from 4.5 to 5.1 over the remainder of its 8500-year history. Low pH values of 4.5-4.7 were found at the end of the Boreal Chronozone and in the Subboreal Chronozone and higher pH values of 4.7-5.1 in the Atlantic and Subatlantic chronozones. This pattern is believed to reflect changes in macroclimate, particularly humidity. The rapid early acidification was shown in the cladoceran succession by the replacement of Bosmina longirostris by Bosmina (Eubosmina) longispina and by an increased number of species such as alonella nana and Alonella excisa. considerable shifts in diatom species composition and abundance were observed during the most recent times, but distributions indicate that all the abundant taxa most frequently occur under acidic conditions, suggesting that other factors in addition to pH may be responsible for the shifts.