Ectodesmata were demonstrated in the outer walls of epidermal cells of French bean following treatment of leaf tissue with Gilson's solution.There was little change in the number of demonstrable ectodesmata with increasing age of the plant or as a result of increased suction tension of the culture solution. The number of ectodesmata was greater after darkening for 24 h but was not increased further with an additional period of darkening.These results are discussed in relation to studies on susceptibility to plant viruses and the structural evidence for ectodesmata. It is suggested that ectodesmata do not determine susceptibility to plant viruses and are not equivalent to the infectible site.