Among approximately eighty larvae of Trachea secalis, gathered from the same locality near Cape Town, South Africa, the incidence of microsporidian infection has been found to be 100%. In the majority of cases the parasite had heavily involved the lower portion of the gut, whereas the rectum and the upper portion of the gut were never infected. In severe cases the gut tissue was practically destroyed and replaced by an almost solid mass of spores. Except during the stages preceding spore formation, the infected portion of tissue can be detected by the unaided eye. When spores have been produced in large numbers, the involved area, which may be no larger than the head of a pin, and is usually located within the posterior third of the gut, becomes white and opaque, and contrasts sharply with the healthy surrounding tissue. The latter is transparent, and appears dark, owing to the colour of the contents. At a later stage it is not unusual to find the infected zone in the form of a narrow band which encircles the gut. The infection then spreads upwards and downwards. Finally, the entire lower portion of the gut becomes white and chalky. With the exception of the gut contents the parasite has not been found elsewhere in the body of the host.