Silicon accumulation in Matteuccia struthiopteris (L.) Todaro was investigated by electron microscopy and X-ray microanalysis. Silicon levels were highest in petioles of mature vegetative leaves, where it was restricted largely to the walls of epidermal and collenchymatous tissues. The pattern of silicon distribution within these walls was random and at no time were electron-dense aggregates of spherical particles observed. The absence of silicon from immature cells indicates that this element accumulated in vegetative cell walls during final stages of development. Moreover, the lack of silicon in lignified cell walls suggests that conditions suitable for lignification in ferns are incompatible with silicon deposition.