Entamoeba invadens RODHAIN is known as a dangerous parasite in reptiles, especially snakes and lizards. Up till now it has mostly been found in specimens which are kept in captivity. When these are carriers they show no signs of disease, but the faeces contain cysts and can infect healthy reptiles. If the reptiles are ill, the symptoms mostly are serious. They begin with a loss of appetite and an increasing need for drinking water. Within a few weeks the faeces merely consist of bloody mucus, containing a large number of hystolytic forms as well as a few cysts of Entamoeba invadens. In the case of Lacerta agilis (STAM, 1958), the animals died on an average within 25 days from inoculation (14 to 34 days). Different species of Natrix which had been infected died in 13 to 77 days from the onset of infection (RATCLIFFE and GEIMAN, 1938). BARROW and STOCKTON (1960) found that the temperature affected the symptoms in infected snakes. When the animals were kept at 13° C there were no internal pathological changes within two to six weeks but at 25° C these were very clear. These changes, as described by different authors, are ulcers of the colon. In experiments nearly the entire colon is damaged to such an extent that no individual ulcers can be distinguished. Ulcers may also be found in the ilium. The liver may have one or several abscesses. Inflammation sometimes spreads from the gut to the kidneys.