By means of a ligaturing technic, which is descr., the route by which an unidentified sp. of Nosema invades the body cavity of its host was detd. The host in this case was the potato tuberworm, Gnorimoschema operculella. It was concluded that the microsporidian is not capable of invading the body cavity of the tuberworm through the wall of either the foregut or the hindgut, but invasion through the wall of the midgut (and possibly through the Malpighian tubes) takes place regularly. Similar results were obtained in determining the invasion route of the virus of a polyhedrosis of the California oakworm, Phryganidia californica.