We describe a new statistical method for measuring bias in the codon usage table of a gene. The test is based on the multinomial and Poisson distributions. The method is used to scan DNA sequences and measure the strength of codon preference. For E. Coli we show that the strength of codon preference is related to levels of gene expression. The method can also be used to compare base triplet frequencies with those expected from the base composition. This second type of codon bias test is useful for distinguishing coding from non-coding regions.