Using the epilithic diatoms as indicators of environmental conditions, 10 lakes in the High Tatra Mountains were characterised with regard to their stage of acidification. The diatom communities were diverse; 210 taxa were found, including 27 species of north and alpine preference. On the basis of cluster analysis, two groups of lakes were distinguished. The first group included shallow, moderately acidified lakes (Dlugi and Zmarzly Gasienicowy lakes), where acidophilous organisms prevailed, with Achnanthes marginulata being the most numerous. The structure of the diatom community in Dlugi lake was stable over 4 years of observations and did not show any progress in acidification. The second group included mostly very deep, non-acidified lakes where circumneutral organisms prevailed, and among them Achnanthes minutissima was dominant in most cases. The diatom community structures mostly confirmed the status of the lakes studied as judged from their water chemistry.