A type 3 metallothionein cDNA (PMT3a) from ozone-treated Populus alba${ imes}$Populus glandulosa cDNA library has been isolated and characterized. A PMT3a cDNA is 459 nucleotides long and has an open reading frame of 201 bp with a deduced amino acid sequence of 66 residues (pI 4.94). The deduced amino acid sequence of PMT3a matched to the previously reported metallothionein genes. The deduced amino acid sequence of PMT3a showed the $86\%$ identity with P. balsamifera ${ imes}$P. deltoides. Expression of PMT3a by the RT-PCR was increased 60 min than 30 min after drought treatment. The ozone treated poplar increased at 30 min in the early time and then decreased at 60 min.