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Algal community structure on natural substrates of thermally altered artificial streams was studied for one year. The streams were fed by a natural blackwater stream near Aiken, South Carolina. Temperature-related shifts in the abundance of major species were demonstrated. Red algae were absent from a stream heated 12.5°C above ambient, but remained abundant in streams heated 7.5 0 C or less. Water temperatures above 30°C produced blue-green algal dominance and eliminated several indigenous species. Substrate specificity was exhibited by all major taxa. Several taxa were abundant only on the bottom sediments and some showed a distinct affinity for either sand or silt, the principal bottom types.