The base contents of A, C, G or U for the three codon positions and the total coding sequences show positive correlation.

The distribution of the occurrence frequencies of each of the four bases at the first, second and third codon positions and in the total coding sequences is analyzed by a graphic method. It is shown that for the coding sequences of 90 species, A has its largest frequency at the second codon position and the smallest one at the third position. C and U have their least frequencies at the first codon position, while G has its largest frequency at the first codon position. By this method, we also find that for each base, there is positive correlation between every two frequencies of the base in the first, second, third and the total coding sequences for 90 species. For each of the four bases, the correlation between the frequencies at the third codon position and that in the total coding sequences is more prominent than others. A statistical method is used to give a precise description of the correlation for the frequencies of every base and it is found that the conclusions drawn by the graphic method are consistent with that got by the statistical method.