Abstract
Lithophyllum crouanii and Titanoderma pustulatum have been found to be common epiphytes on Laminaria hyperborea stipes from western Norway. The two species were studied by light microscopy and SEM. The main character used to separate the two species macroscopically is that L. crouanii has immersed conceptacles seen as white circles on the thallus surface, whereas T. pustulatum has gradually raised conceptacies. Sporangial conceptacies are uniporate in both species. L. crouanii has zonately divided tetrasporangia and gametophytic plants were common. Only bisporangial plants were found in T. pustulatum. In vegetative specimens a cross section revealed that T. pustulatum has a unistratose hypothallium composed of oblique palisade cells and a bistratose margin. L. crouanii has horizontally aligned perithallial cells of more or less equal size and never has a bistratose margin.