To assess the effects of high temperature stress on membrane permeability in mosses, we studied Plagiomnium acutum (Lindb.) T. Kop. and Thuidium cymbifolium (Doz. & Molk.) Doz. & Molk. in May and June 2001, using changes in conductivity measurements of the medium. The relative membrane permeability increased with the treatment duration within the range of 40–60°C. The higher the temperature, the more quickly the relative permeability increased. Regression curves indicate that the relationship between the relative percent permeability and temperature are S-shaped and fit a logistic model. The 50% injury temperatures (IT₅₀) are 44.1 and 45.3°C, respectively.