Sensitivity to salinity varied widely among 12 Californian lichen species occurring along a coastal to inland gradient, with the degree of sensitivity being approximately proportional to the distance from the coast where the species grew. For four of those species, it was found that net photosynthesis in three decreased with increasing sea-salt concentrations, but in Dendrographa minor Darb. it was essentially unaffected. Time-course studies with Pseudacyphellaria anomala Magn., a sensitivity species, demonstrated that a major decline in net photosynthesis occurred within an hour after submersion in a full strength sea-salt solution and that the effect was partially reversible, but only up to 12 h after initiation of the treatment. For three sensitive species, an osmotic effect provided by treatment with sorbitol only partially explained the observed changes in gas exchange. Solutions of NaCl of equal osmotic value to full-strength sea-salt solutions decreased net photosynthesis more than full-strength sea-salt solutions alone. Because NaCl provides the principal ions in sea-salt solutions, it is inferred that the observed ionic effects were principally due to these ions. Of the sensitive species Evernia prunastri (L.) Ach. was least sensitive to osmotic effects alone, but it was extremly sensitive to salinity.