Two dibenzofurans, isostrepsilic acid and hypostrepsilic acid, were pr
oduced in large quantities by the mycobiont of Usnea orientalis cultur
ed on a malt-yeast extract medium containing sugar alcohols. The natur
al lichen growing in the field produces usnic and salazinic acid but n
ot the above dibenzofurans. Isostrepsilic acid is a new product which
has shown to be y-9-hydroxymethyl-1-methydibenzofuran-2-carboxylic aci
d. It seems that the production of these compounds is probably due to
the combined effects of the high osmotic pressure of the medium and th
e nutritional conditions. The production of the dibenzofurans was mark
edly repressed by actively photosynthesizing photobionts.