Tenuazonic acid which was reported hitherto from Alternaria tenuis and A. longipes has now also been found in A. kikuchiana and A. mali, during our investigations on the host-specific toxins in their culture filtrates. However, further investigation on the 19 different isolates of A. kikuchiana has revealed that tenuazonic acid was produced not in all isolates used, but in only 6 isolates. A similar investigation on A. mali showed that its productivity was in different proportions with the isolates. All virulent isolates of both the Alternaria spp. are well-known to produce a host-specific toxic filtrate, in which so-called host-specific toxin is contained. Tenuazonic acid was fairly toxic to young leaves of Japanese pear and apple which are host plants of both the fungi respectively, but not involved in a host specificity of culture filtrates, because both the resistant and susceptible cultivars of each plants were equally sensitive to tenuazonic acid. Among the isolates used, however, tenuazonic acid production was frequently found to be in harmony with that of host-specific toxin in culture filtrate. Screening of 185 isolates, belonging to several species of Alternaria, for tenuazonic acid production revealed its widespread occurrence in this genus. Results suggesting tenuazonic acid as a characteristic metabolite and not a pathogen toxin are also discussed in relation to chemotaxonomical interest.