Alternaria brassicae (Berk.) Sacc., a necrotrophic fungus devastating oilseed Brassica crops in India, causes up to 47% reduction in seed yield. Morphological characteristics of different isolates revealed variation in growth, shape and pigmentation of colony, conidial measurements and number of septa. Conidial length varied from 106.7 to 285.9 µm, width 33.5 to 57.0 µm and beak length 41.4 to 180.0 µm. Number of horizontal septa varied from 3.2 to 8.0 and vertical 0.3 to 1.4. Different synthetic media showed profound variation in mycelial growth of A. brassicae isolates and the poor sporulation indicated that the fungus requires some organic sources of nutrition for better growth and sporulation. The degree of sporulation of A. brassicae isolates is a function of nutrition proved for the first time. Per cent inhibition of mycelial growth showed diverge among A. brassicae isolates, which may be due to the variation towards fungicidal sensitivity among isolates. Pathogen aggressiveness study demonstrated the existence of considerable variation in tolerance of Brassica species to A. brassicae, which is proved with the location specific disease severity.