Codon usage bias,as a combined interplay from mutation and selection,has been intensively studied in Escherichia coli.However,codon usage analysis in an E.coli pangenome remains unexplored and the relative importance of mutation and selection acting on core genes and strain-specific genes is unknown.Here we performed comprehensive codon usage analyses based on a collection of 26 complete genome sequences of E.coli.Our results show that core genes that are present in all strains have higher codon usage bias and stronger translational selection when compared to other genes.