Codon usage in Plasmodium vivax nuclear genes was analysed and compared with that in Plasmodium falciparum nuclear genes. Preferred codons were determined for P. vivax. Unlike P. falciparum, P. vivax genes are about 15% less A+T rich in the coding regions, with no obvious A+T bias at the third position of the codons. The amino-acid composition of P. vivax gene products is also different from that of P. falciparum. These results provide valuable information to facilitate gene cloning as well as expression and transfection studies for P. vivax.