In prokaryotes, GC levels range from 25% to 75%, and Topt from approximately 0 degrees C to >100 degrees C. When all species are considered together, no correlation is found between the two variables. Correlations are found, however, when Families of prokaryotes are analysed. Indeed, when Families comprising at least 10 species were studied (a set of 20 Families), positive correlations are found for 15 of them. Furthermore, a comparative analysis by independent contrasts made within the Families in order to control for phylogenetic non-independence showed qualitatively equivalent results. We conclude that Topt is one of the factors that influences genomic GC in prokaryotes.