The present study describes the taxonomic composition, richness and seasonal dynamics of the diatom community from thermal-sulphur waters of "Fiume Caldo" (North-western Sicily). The diatom flora of these waters consists of 64 taxa, showing similarities with thermal diatom communities from different geographical areas. Among the recorded taxa, Diadesmis confervacea Kütz. is a new record for Sicily. Seasonal changes in assemblage composition and richness have been recorded; the highest diversity values were observed during winter and spring, while during summer the diatom community consisted of a limited number of organisms whereas blue-green algae were quantitatively dominant. Inside the studied community several species showed a preference for certain substrata. Most diatom taxa were classified as alkaliphilous, β-mesosaprobic and eutraphentic according to their affinity for pH, saprobity and trophic state. Some taxa were considered very representative of this habitat and therefore been identified as the characteristic group of species for these waters.