Lamerin breed of the silkworm Bombyx mori L. is the only mulberry silkworm breed which survives with an associated microsporidian $(Lb_{ms})$ infection from generations. This present comparative investigation has produced information on the influence of $Lb_{ms}$ on the growth and development of the breed. The $Lb_{ms}$ transmits tranovarially as well as perorally at low level. Larvae developed from the eggs laid by female, infected with $Lb_{ms}$ were as normal as larvae developed from the eggs laid by healthy female. In infected tissues early developmental stages and mature spores located intracellularly but did not show hypertrophy of cells of the host. The meronts and sporonts measures $0.46{pm}0.25,;1.11{pm}0.05{mu}m$ in length and $0.32{pm}0.05,;0.78{pm}0.10{mu}m$ in width respectively. The mature spore measures $4.36{pm}0.06{mu}m$ in length and $2.14{pm}0.01{mu}m$ in width.