Two years ago, we showed that positive correlations between optimal growth temperature (T-opt) and genome GC are observed in 15 out of the 20 families of prokaryotes we analyzed, thus indicating that "T-opt is one of the factors that influence genomic GC in prokaryotes". Our results were disputed, but these criticisms were demonstrated to be mistaken and based on misconceptions. In a recent report, Wang et al. [H.C. Wang, E. Susko, A.J. Roger, On the correlation between genomic G+C content and optimal growth temperature in prokaryotes: data quality and confounding factors, Biochem. Biophys. Res. Commun. 342 (2006) 681-684] criticize our results by stating that "all previous simple correlation analyses of GC versus temperature have ignored the fact that genomic GC content is influenced by multiple factors including both intrinsic mutational bias and extrinsic environmental factors". This statement, besides being erroneous, is surprising because it applies in fact not to ours but to the authors' article. Here, we rebut the points raised by Wang et al. and review some issues that have been a matter of debate, regarding the influence of environmental factors upon GC content in prokaryotes. Furthermore. we demonstrate that the relationship that exists between genome size and GC level is valid for aerobic, facultative, and microaerophilic species. but not for anaerobic prokaryotes. (c) 2006 Elsevier Inc. All rights reserved.