The characterization of type cultures of the S. thermophilum complex remains unclear and morphological features have been relied upon to recognize such isolates. The aim of this study was to compare morphological and thermal methods, in order to identify relationships between cell wall and cultural characteristics. The differences in the proportions of structural and non-structural fractions and the peak decomposition temperatures of the test isolates were significant. The thermograms produced can be grouped into two distinct types, which can be further sub-divided, in agreement with microscopic classification. The relationship between the various morphological parameters including extension rate, sporulation, microscopic type and DTG parameters are discussed briefly.