The psbA gene cloned in pSB135 from Solanum nigrum atrazine-resistant biotype was sequenced. It has the same nucleotide sequence as the known atrazine resistance gene from another independent biotype of S. nigrum. On the basis of the deduced amino acid sequences the secondary structure of the 32kD proteins encoded by the psbA genes were compared between atrazine-resistant and susceptible biotypes, and some indications from the protein structure comparison were discussed.