Temperature preferences and small-scale distribution patterns of four Dermatocarpon taxa in the catchment areas of two Bulgarian rivers were analyzed. Taxa with an overall alpine or arctic-alpine distribution were restricted to microhabitats with the lowest water and rock surface temperatures. A trend for increasing Dermatocarpon rivulorum thallus diameter and colony size (aggregates of thalli) with decreasing temperature was identified. Data on pH and conductivity of the water for the study sites are provided for all taxa. Dermatocarpon arnoldianum is reported for the first time from Bulgaria, confirmed by ITS sequence data, and its known range extends to South-East Europe. All previous records of D. rivulorum from Bulgaria were based on misidentified specimens of D. arnoldianum. Freshwater taxa of Dermatocarpon are proposed as a tool for monitoring the biological effects of temperature changes due to global climate change. Thallus and colony size are recommended as additional features for monitoring populations of threatened Dermatocarpon species.