SUMMARYSuccessful resyntheses of theCladonia cristatellamycobiont with its normal phycobiont and withTrebouxiaisolates from 12 other species of lichens all produced the characteristic depside barbatic acid as the major secondary product. Many also produced the dibenzofuran didymic acid, but none yielded detectable amounts of the cortical pigment usnic acid. No characteristic lichen products were found in cultures of theC. cristatellamycobiont either alone, under a variety of cultural conditions, or mixed withPseudotrebouxiaphycobionts under the same conditions that led to successful resyntheses withTrebouxiaphycobionts. Well-developed soredia from successful resyntheses of theLecanora chrysoleucamycobiont withPseudotrebouxiaphycobionts showed traces of usnic acid. The production of characteristic lichen products is affected primarily by the establishment of a successful symbiosis and secondarily by as yet poorly defined environmental factors. The formation of many characteristic lichen products may involve algal inhibition of fungal enzymes.