The nucleic acid sequence bank now contains over 600 protein coding genes of which 107 are from prokaryotic organisms. Codon frequencies in each new prokaryotic gene are given. Analysis of genetic code usage in the 83 sequenced genes of the Escherichia coli genome (chromosome, transposons and plasmids) is presented, taking into account new data on gene expressivity and regulation as well as iso-tRNA specificity and cellular concentration. The codon composition of each gene is summarized using two indexes: one is based on the differential usage of iso-tRNA species during gene translation, the other on choice between Cytosine and Uracil for third base. A strong relationship between codon composition and mRNA expressivity is confirmed, even for genes transcribed in the same operon. The influence of codon use of peptide elongation rate and protein yield is discussed. Finally, the evolutionary aspect of codon selection in mRNA sequences is studied.