Microsporidan infection in larval trematodes is primarily an infection of the wall of the germinal sac (sporocyst or redia), spreading from there to the contained developing embryos, causing bloating, distortion, and destruction of the embryos. The end result of Nosema strigeoideae infection in strigeoids is a sporocyst filled with cellular debris, deformed cercariae, spores, etc. In echinostomes infected with Nosema echinostomi the rediae become solidly packed with spores confined within a reticular network which almost completely obliterates the cavity. It is suggested that these differences may be due to morphological differences in the larval trematodes.