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<i>Gaeumannomyces</i> <i>graminis</i> var. <i>graminis</i> (Ggg) is an ectotrophic root infecting fungus that is the causal agent of take-all root rot of St. Augustinegrass (<i>Stenotaphrum secundatum</i>) and root decline of other warm-season turfgrasses. Ggg is also the etiological agent of crown sheath rot of rice (<i>Oryza</i> <i>sativa</i>). Colony morphologies of 76 Ggg isolates from St. Augustinegrass throughout Texas were examined on potato dextrose agar. Three distinctive phenotypic groups were found, where 10.5, 80.3, and 9.2% of the isolates were highly melanized with a round colony formation (termed M group), non to slightly melanized with a round colony formation (termed L group), and slow-growing and highly melanized exhibiting an irregular or filamentous colony formation (termed H group), respectively. The M group grew faster at 30°C and was significantly more aggressive than L and H groups based on a rice seedling pathogenicity assay. A maximum likelihood phylogenetic analysis based on partial sequences of internal transcribed spacer and large subunit of the ribosomal DNA, supported that M, L, and H groups were separated into different clades and were associated with other Ggg sequences retrieved from the NCBI GenBank database. M, L, and H groups were also distinct from <i>G. graminis</i> varieties <i>tritici</i> and <i>avenae</i>. To our knowledge, this is the first study to describe the population diversity of Ggg with regards to morphological and biological characterization and genetic background.