Codon usage variation was studied in the parabasilid protozoan Trichomonas vaginalis. Correspondence analysis was employed to identify the single largest source of variation in the dataset. The G+C content at the third position of synonymously variable codons was also evaluated. A strong positive correlation was observed between the effective number of codons measure and the position of the genes on the axis of greatest dispersion following correspondence analysis. The correlation between base composition at the third position and effective number of codons was weaker. The results indicate that the major influence on codon usage bias is translational selection, with a moderate effect attributable to mutational pressure. In addition, twenty codons were identified whose use in biased genes is statistically significantly greater than in unbiased genes.