To identify genes involved in arsenite [As(III)] tolerance, we screened for As(III)-sensitive mutants using ethyl methanesulfonate (EMS)-mutagenized seeds of Arabidopsis thaliana. One mutant with high sensitivity to As(III) was isolated from the screening. It had a mutation in Glu52 of phytochelatin synthase 1 (AtPCS1), and introduction of the wild-type AtPCS1 gene rescued the phenotype, confirming that the mutant represented a new allele of atpcs1 (termed as cad1-7). The expression of AtPCS1 from cad1-7 and cad1-3, a second allele of atpcs1, complemented As-sensitive yeast, but not plants. Our study suggests that AtPCS1 activity is differentially regulated between plants and yeast.