Based on complete bacterial genome sequence data, we demonstrate a correlation between bacterial chromosome length and G+C content of the genome, with longer genomes having higher G+C contents. The correlation value decreases at shorter genome sizes where there is a wider spread of G+C values. However although significant (p < 0.001) the correlation value (Pearson R = 0.58) suggests that other factors also have a significant influence. A similar pattern was seen for plasmids; longer plasmids had higher G+C values, although the large number of shorter plasmids had a wide spread of G+C values. There was also a significant (p < 0.0001) correlation between G+C content of plasmids and the G+C content of their bacterial host. Conversely, the G+C content of bacteriophages tended to reduce with larger genome sizes and although there was a correlation between host genome G+C content and that of bacteriophage it was not as strong as that seen between plasmids and their hosts.