Infection of maize stigmas (silks) by the smut fungus Ustilago maydis was documented with micrographs. Stigmas were inoculated with different aqueous suspensions of smut sporidia. When inoculum consisted of individual haploid strains, no mating or infection structures were observed. When stigmas were inoculated with sporidia that had compatible alleles at both mating loci (a and b), sporidia mated in pairs using a conjugation tube, and each pair formed a dikaryotic infection hypha that grew rapidly across the stigma surface, developed an appressorium, and entered the stigma. When sporidia were compatible at a but not at b, mating occurred irregularly, and resultant hyphae grew slowly and did not enter stigmas [...]