Clones of zinc-tolerant and non-tolerant genotypes of Betula pendula Roth, and B. pubescens Ehrh. were cultured aseptically on media containing different concentrations of zinc. The location, concentration and form of zinc in root tissues of both groups of genotype were compared using various microanalytical techniques. At concentrations of zinc at which all genotypes were viable, there were no obvious differences between the tolerant and non-tolerant genotypes. When the roots were subjected to concentrations above the lethal threshold, zinc accumulated intracellularly, at the endodermis, in the form of electron-dense granules. This form of accumulation was symptomatic of tissue inundation with zinc. The results do not support the presence of cell wall binding or intracellular detoxification mechanisms of tolerance. The possibility that zinc tolerance, control of zinc uptake and zinc-induced inhibition of cell extension are all linked by common events centred on the plasma membrane are discussed.