Following studies with whole cells, the identification of the succinate dehydrogenase complex of the respiratory electron transport system as the site of action of carboxin and related carboxamides was established by experiments with mitochondrial preparations. Carboxamides appear to be most active and possibly selective toward Basidiomycete fungi. At one time, in fact, it was thought that the molecular site of action for carboxin might be unique to Basidiomycetes. There are abundant possibilities for future research with carboxamides not only at the molecular level, but, from a practical viewpoint, studies directed at the extension of the fungitoxic spectrum to non-Basidiomycetes and control of mutational resistance to commercially used carboxamides. It is interesting that some diverse types of carboxamide structures have been found which give weak inhibition of succinate dehydrogenase complex activity, yet are strong inhibitors of fungal growth. This situation could indicate multiple reaction sites and/or metabolic conversion of the carboxamides to an active metabolite.