Nucleotide variation in the ACL5 gene region, which encodes spermine synthase, was analysed for 21 Arabidopsis thaliana ecotypes and one accession of Arabis gemmifera. In Arabidopsis thaliana, dimorphism was also detected in the ACL5 region, as in other nuclear genes of this plant. The nucleotide diversity ( pi ) of the entire region, exon and intron was 0.0163, 0.0042 and 0.0293, respectively.