Abstract: Trichoderma harzianum, T. viride, T. virens and Aspergillus nigerwere isolated from mustard leaf surface. Efficacy of their conidial suspension and culture filtrates was evaluated. In dual culture test, T. viride caused maximum inhibition of radial growth of A. brassicae (74%) and A. brassicicola (77%). Highest concentration (3:1) of T. viride culture filtrates caused 60 and 54% inhibition of radial growth of A. brassicae and A. brassicicola, respectively. The culture filtrates of T. viride at their highest concentration (3:1) caused 71 and 69% inhibition of conidial germination of A. brassicae and A. brassicicola, respectively. In field trial foliar spray of 1x107 conidia mL-1 of biological control agents was done on mustard plants 24 h after artificial inoculation by A. brassicae conidial suspension (1x105 conidia mL-1. Viability of BCAs was recorded up to 30 days in phylloplane of mustard at relative humidity between 80-90% and temperatures between 20-25°C. T. viride proved to be most effective as its conidial suspension caused up to 76 and 68% reduction in disease intensity on leaves and pods, respectively. Whereas, its culture filtrate caused 45 and 36% reduction in disease severity on leaves and pods, respectively. These results suggested that T. viride can be used to control Alternaria blight disease of mustard.