The amino acid composition of more than 20 enzymes and protein from various closely related mesophilic and thermophilic micro-organisms (esp. Bacillus) have been used to calculate a variety of macroscopic parameters. These included the hydrophobic index (H phi ), the ratio of polar to non-polar volumes (rho), the ratios of Arg/(Arg + Lys), and (Arg + Lys) or (Glx + Asx) to total amino acids, % H-bonding amino acids, % alpha-helix- or beta-sheet-forming amino acids, the theoretical melting temperature (TCalcm), the total volume to total amino acid ratio (VR), and the % non-polar residues (NPS). In contrast to previous similar comparisons with proteins from divergent sources, it was found that thermophilic vs mesophilic proteins from the same genus show correlations between thermostability and increased H phi, decreased rho, and increased Arg/(Arg + Lys), as well as increased alpha-index and beta-index. Weaker correlations were seen for VR, TCalcm, aliphatic index, and NPS, all derived from, or related to, H phi. No correlations existed for the other calculated parameters. These results are consistent with recent results of Argos et al. (1979) [Biochemistry 18, 5698-5703] on sequence analyses of glyceraldehyde-3-P dehydrogenases, where thermophilic proteins showed multiple amino acid replacements that caused increased internal hydrophobicity and increased external polarity. No trends were observed in any of the parameters calculated from amino acid compositions for crude cytoplasmic protein extracts from mesophilic vs thermophilic Bacilli.