A large number of new analogs of 2-methylbenzanilide (I; o-toluanilide) and structurally related carboxamides were synthesized and tested for inhibitory effects on Complex II (SDC) activity in mitochondria from sporidia of wild-type and carboxin-selected mutant strains of Ustilago maydis(corn smut). The data complements and extends that obtained previously with substituted benzanilides [Pestic. Biochem. Physiol.27, 249–260 (1987)]. Certain 3′-substituted analogs of (I) such as 3′-benzyloxy-2-methylbenzanilide, were highly active inhibitors of both wild-type and mutant enzyme complex activity. Substitution of N-alkyl groups for the phenyl ring of (I) produced active compounds, e.g., N-1,5-dimethylhexyl, N-n-dodecyl, and N-n-tetradecyl analogs. Phenyl replacement by a variety of ring systems gave low inhibition, with the exception of the caged adamantane structure. Apparent selective inhibition or specificity for the carboxin-resistant SDC was shown by a number of analogs, primarily 4′-substituted derivatives of (I). For instance, the 4′-n-valerophenone analog of (I) was 13 times less active than the parent anilide on the wild-type SDC and 16 times more active than (I) on the mutant SDC. Structure-activity results for an assortment of miscellaneous heterocyclic carboxanilides revealed compounds selectively active against the mutant SDC. These included the 4′-n-hexyl and 4′-phenoxy analogs of 1,4-dihydro-2-methylbenzanilide and the 4′-methyl and 4′-i-propyl derivatives of 2-chloropyridine-3-carboxanilide. N-Methylpyrrole-2-carboxanilide and 1,4-dihydro-2-methylbenzanilide which lack a double bond between the methyl and carboxanilido groups on the heterocyclic ring were fairly active, showing that a cis-crotonanilide structure is not necessarily a basic requirement for inhibition. Inhibition of the wild-type U. maydisenzyme complex was generally mirrored by a similar inhibition of R. solanigrowth. Some exceptions were encountered with diverse compounds such as the 4′-ethyl, N-1-methyl-2-phenoxyethyl and 4-methylthiazol-2-yl analogs of (I) which gave strong inhibition of R. solanigrowth but weak inhibition of SDC activity.