Cultured lichen tissues (composed of a mycobiont and a phycobiont) were obtained from fresh thalli of Cladonia vulcani Savicz (collected from Ebino, Kyushu) by means of the method described by Yamamoto and Yamada. Trebouxia excentrica Archibald was isolated from the cultured tissues. The following different pH media were prepared by adding sulphuric acid or caustic soda to modify the malt-yeast extract culture media : 2 , 4 , 5.5 , 7 and 9. About 5mg cultured tissue and algal colonies were established on the defferent pH media. Each cultured tissue colony was weighed and transplanted every 4 weeks ; each algal colony was similarly treated. Growth curves cultured tissue in different pH media are presented, as are groth curves of symbiotic algal colony in different pHs. Growth ratios (fresh weight after 12 weeks culture / initial weight) of cultured lichen tissues and symbiotic algal colonies in different pH media are determined : cultured lichen tissues of Cladonia vulcani grow best in pH 4 media, while symbiotic algae (Trebouxia excentrica) grow well in media between pH 4 and pH 9. The soil of natural habitats of C. vulcani shows pH 5.5,which is the same growth ratio of mycobionts and phycobionts; this may suggest that the formation of lichen thalli needs just same growth ratio of both mycobionts and phycobionts and it may be controlled by the habitat pH. This is the first record of Trebouxia excentrica Archibald from Japan as well as outside of Europe and North America. [in Japanese with English abstract]