In March, 2007, we carried out the survey of microsporidian infection in the common cutworm, Spodoptera litura distributing in the southern area of Vietnam. Of the microsporidian strains isolated from the adult collected, it selected the 12 strains which exhibited the high infectivity to the larvae of S. litura. One strain (Nosema sp. VSl-2007-16: it named as No. 16 strain tentatively) of them was regarded as a strain of Nosema bombycis using analysis of SSU rRNA sequence. To clarify the differences of infectivity due to the various host insects in this study, it was compared with the mechanisms of infection and multiplication in the larvae of S. litura and Bombyx mori, and insect tissue cultured cells derived from S. frugiperda (Sf 9 cells) and cultured cells from B. mori (BmN4 cells). No. 16 strain was repeated the continuous infection and multiplication in the larva of S. litura and Sf 9 cultured cells. On the other hand, it revealed that No. 16 strain did not multiply due to the arrested development of sporoplasms in the larva of B. mori and BmN4 cultured cells after invasion of them derived from the hatching of spore. Therefore, No. 16 strain exhibited the characteristic developmental stages of microsporidia belonging to genus Nosema. In conclusion, it was considered that the differences of infectivity of No. 16 strain were concerned in the variation of developmental pattern in each insects. From the analysis of SSU rRNA sequence in previous report and the observation of developmental stages of this strain, it suggested that No. 16 strain was a non-infectivity N. bombycis strain to B. mori. For the reason of this strain has the host specificity which showed the infectivity to S. litura and non-infectivity to B. mori, it may be possible to apply as a microbial control agent to S. litura in future.