Magnaporthe oryzae, the cause of leaf and panicle blast, is the most damaging rice pathogen worldwide, causing yield severe losses. Blast management is based mainly on the application of fungicides; however, the sensitivity of the pathogen populations to the main fungicides is largely unknown. More or less sensitive phenotypes were identified when exposed to the fungicides most used for disease control in Brazil such as azoxystrobin, tricyclazole, trifloxystrobin + tebuconazole and tebuconazole. About 90% of the genotypes collected in Tocantins State (where disease is usually most severe) were resistant to azoxystrobin, based on the detection of G143A mutation (resistance marker for QoIs). The EC₅₀ value for these isolates was >10?g. L^-1. Most of these resistant isolates were significantly less fit than the sensitive ones when inoculated in plantaas measured by the respective latent periods, infection efficiencies, sporulation intensities and blast severities. The proportion of resistant and sensitive isolates in mixed inoculum of rice plants remained constant in the presence of azoxystrobin for four generations. Conversely, in the absence of the fungicide, the resistant populations wereprogressively reduced. While mutants with multiple resistances to diverse active ingredients in different fungicide groups may also arise, the fitness costs associated with resistance to more than one active ingredient may impair competitive abilities of such mutants even further.