Four strains of silkworm Bombyx mori L., i.e. J-101, 206MKD, C-102 and 207-PO were evaluated for their resistance to microbial diseases. For each strain 225 larvae of 4th instar were separated from mass rearing lot and divided into three replications. The rearing of silkworm was carried out at 25 ± 2°C and 70 ± 5% rH. The results revealed that bacterial flacherie caused overall 20.52, 18.5, 16.82 and 5.53 percent mortality in C-102, J-101, 206-MKD and 207-PO, respectively. The mortality did not differ significantly among C-102, J-101 and 206-MKD. Viral grasserie resulted overall 16.65 6.95, 5.52 and 1.87 percent mortality in J-101, C-102, 206-MKD and 207-PO, respectively. There was non-significant difference of mortality between 206-MKD and C-102. White muscardine was not found in 5th instar larvae as well as in spinning larvae; however it caused maximum 2.99% mortality in J-101 during pupil stage. Cumulative mortality because of these three diseases was in the array of J-101 (38.59%) > C-102 (29.22%) > 206-MKD (23.67%) > 207-PO (9.93%). Thus 207-PO was relatively the most resistant strain to flacherie and grasserie while J-101 was relatively the most susceptible strain to these microbial diseases. 206-MKD and C-102 were intermediate between 207-PO and J-101, however they were relatively more resistant to grasserie than flacherie.