Due to an infection of the nutrient medium with moulds there was a significant (p = 0.05) increase in the length, width and gibbosity of Lemna gibba L. plants. The Lemna gibba plants, which had been collected in the field in the flat form (gibbosity < 1.00 mm), were cultured in 100 ml Erlenmeyer flasks in the presence of the chelating agent EDDHA. The culturing conditions and the composition of the nutrient medium were according to De Lange & Pieterse (1973). Sterilization of plants and medium was carried out in the usual way, but part of the cultures became infected in the growth cabinet. Table I gives the dimensions of the infected and non-infected plants. In the light of the findings of Pieterse (1976) and Elzenga et al. (1980) the observed phenomenon might be explained by the fact that moulds can produce ethylene (cf. Young etal. 1951).