This present investigation was carried out to study the variation at the b mating type in the basidiomycete fungus  Ustilago maydis, the causal agent of corn smut disease. Different isolates were collected from: Assiut, Sohag, Menia, Sharqiya, Daqahlia,Giza and Kalubia governorates in Egypt. Eight different b mating types were identified among the samples with approximately equal frequencies. The b₁ , b₃ and b₈ alleles were the most frequent, whereas the alleles  b₄ and b₆ were the least frequent. The allele b₄ was absent in Sohag samples whereas the allele b₆ was absent in Assiut, Menia and Lower Egypt samples. The gene diversity G_st (population differentiation) for b mating types within and between subpopulation were calculated. The total diversity over all populations was 85.86 and the average gene diversity within populations was 82.79 and the gene diversity among population was 3.07 indicating that the majority of gene diversity was occurring in small spatial scale. Low population differentiation was indicated by the low G_ST values among populations, suggesting corresponding high degree of gene flow in these population. Ten U. maydis samples collected from four geographic regions (Assiut, Sohag, Menia and Lower Egypt) and belonging to different b mating types had been selected for RAPD analysis. Such analysis generated 40 discrete DNA bands ranging from 150-2500 bp in size. Twenty seven of which were polymorphic (67.50), with an average of  8  bands per primer.    One putative a₂ mating type specific band was generated with primer OPO-12 of 2400 bp in size.      Results indicated that no relationship between the RAPD profile and geographic sites where isolates were collected from.