Some aspects of the autecology of Mesocoelium monodi Dollfus, 1929, a parasite of lizards and Amphibia in Africa have been studied because of the dearth of knowledge in this field of ecology. It was found that although the larvae were confined to one species of molluscan host, the adults occurred in a wide variety of taxonomically diverso hosts including reptiles and amphibia. The possible reasons for this this low order of host specificity are discussed. Although the parasite occurred in all parts of the alimentary canal of Agama with the exception of the rectum, its preferred microhabitat was the duodenum where it feeds on food detritus and mucus. M. monodi does not appear to produce any tissue reactions and the lizards do not acquire a durable immunity or premunition to it. There is no evidence of intra- specific and interspecific competition taking place within this microhabitat as most of theavailable gut space was not utilizedand Mesocoelium and Physaloptera, the other helminth occurring in the duodenum, did not appear to be mutually exclusive. The frequency distribution of the parasite in the lizards is not random and it is suggested that the main reason for this is that there are foci of infestation. The incidence and intensity of infestation which are controlled by the rate of ingestion of intermediate hosts containing metacorcariae vary seasonally and arc greater in females than in males. Tho most important climatic factor influencing the seasonal changc is rainfall, probably because it controls the activity of both the molluscan intermediate host and the infective larvae. The adult parasite has a short life span and the mortality rate was about 1.5 to 2.5 per cent per day.