Ustilago maydis is the fungus causative of common smut in maize and teozintle. Under natural conditions, the dikaryon formed by mating of sexually compatible strains, penetrates the host plant, and induces the typical disease symptoms: chlorosis, increased synthesis of anthocyanins and formation of galls full of teliospores. In the older literature it was indicated that, under normal conditions, the haploid forms of the fungus were also capable to invade the host tissues, although the symptoms observed were almost nil.Since previous data from our lab showed that that haploid strains of the fungus were able to infect non-natural hosts under axenic conditions, in the present communication we proceeded to determine whether haploid strains caused more serious symptoms when infecting maize plants under axenic conditions. It was observed that indeed U. maydis haploid strains were able to invade maize plants under both axenic or soil conditions, and induces chlorosis and increased anthocyanin formation, although, as expected, there occurrence has no formation of galls or teliospores. Production of reactive oxygen species, cell death, necrotic areas, salicylic and jasmonic acid, were higher in axenically infected plants.These results demonstrate that haploid strains of U. maydis are able to infect maize plants and suggest that plants infected in non-sterile soil probably develop general resistance mechanisms, and are accordingly less sensitive to infection than plants infected under axenic conditions.