Abstract: A microsporidian was isolated from Athetis lepigone(Möschler) for the first time and the pathologic changes of infected midgut cells was observed by the transmission electron microscope. The results showed as follows:after infection, the microvilli of midgut cells and cytoplasm ecptoma of goblet cells became swelling and droped; obvious deformation, such as expansion and lengthen occurred in some of the cell nucleus, but no microspore was found in the nucleus; some of the mitochondria also became swelling and out of shape, and degraded in the late stage of infection, the ridges of them changed the arrangement as well; the rough endoplasmic reticulum broke into small pieces and the ribosome attached to them came off gradually. Microsporidia spreaded all over midgut cells at late stage of infection, all kinds of organelles in cells degraded severely. According to the results and predecessors' achievements, we considered that the pathological process started with the changes of the midgut cells, led to cell rupture, extended to the host tissues and organs, lost their function and eventually death.

Key words: Athetis lepigone, microsporidia, ultrastructure, pathological mechanisms