The habitat of the acido- and thermophilic red algae Galdieria sulphuraria and Cyanidium caldarium was examined in acidic hot sulphur springs in the vicinity of Naples (Italy). These species grew on soil and rocks, but a large part of the populations was cryptoendolithic. The endolithic algal layer (1–3 mm in thickness) was covered by amorphous silica (1–2 mm in thickness) containing traces of hydrotroilite (FeS.nH2O) and elemental sulphur. Organotrophic bacteria and fungi were not found in the algal layer. Light penetration measurements showed that 0·1–1% of the sunlight reached the upper part of the algal layer. Thus, low-light-adapted algae should be able to perform some photosynthesis in this endolithic habitat. Under conditions where light is even more limited, e.g. in winter or after darkening of the covering layer, many of the cells might not survive. Aqueous extracts of these algae are excellent growth substrates for Galdieria sulphuraria. Therefore, we propose that compounds released from dead cells in the endolithic layer are used by the surrounding Galdieria cells for heterotrophic metabolism. This would increase their chance of surviving prolonged periods under detrimental conditions.