Revealing the determinants of codon usage bias is central to the understanding of factors governing viral evolution. Herein, we report the results of a survey of codon usage bias in a wide range of genetically and ecologically diverse human RNA viruses. This analysis showed that the overall extent of codon usage bias in RNA viruses is low and that there is little variation in bias between genes. Furthermore, the strong correlation between base and dinucleotide composition and codon usage bias suggested that mutation pressure rather than natural (translational) selection is the most important determinant of the codon bias observed. However, we also detected correlations between codon usage bias and some characteristics of viral genome structure and ecology, with increased bias in segmented and aerosol-transmitted viruses and decreased bias in vector-borne viruses. This suggests that translational selection may also have some influence in shaping codon usage bias.