Observations have been made on the effects of different storage treatments, either 100°0 or 0°0 r.h., 12 h light:12h dark or 24 h dark for 2 days, on the ultrastructure of the phycobionts of Lobaria amplissima and Lasallia pustulata. After storage, Myrmecia, in the more desiccation-sensitive lichen L. amplissima, showed an increased number of cytoplasmic storage bodies which had more frequently dark contents and a half-moon-like appearance.Trebouxia, from the desiccation-resistant L. pustulata, showed major cellular disorganization when stored at 100°0 r.h. in the light:dark regime, whereas, in the dark, reorganization of thylakoids into a parallel arrangement occurred. Treatment at 0°0 r.h. in the light:dark regime induced an accumulation of starch around the pyrenoid of Trebouxia and, in the light:dark and dark regimes, a peripheral location of pyrenoglobuli without a decrease in the total volume of lipid material. The results are discussed in terms of the utilization and mobilization of reserve substances and the habitat preferences of the two lichens.