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The snail Melanoides tuberculata has been used successfully in the control of some snails, intermediate host of parasitosis. Melanoides tuberculata has been found in Tunisia, but its effects on populations of native snail have not yet been evaluated in the field. Our objective is to determine whether M. tuberculata competed with Galba truncatula, using a field study. Twelve monthly investigations were carried out along an irrigation system in Ain Soltane’s oasis (southwest of Tunisia). Here, we describe the population dynamics of G. truncatula with and without M. tuberculata in two stations: a witness pilot station (S1), in which the snail lives alone, and an experimental station (S2), where two species live together. The abundance of G. truncatula varied according to subsistence or not of M. tuberculata. The number of annual generations is higher in S1 (four generations) than S2 (two generations). In the absence of other molluscs or predators that can influence the density of G. truncatula and assuming that climatic factors are mitigated by the presence of an oasean microclimate, these results show that the mollusc M. tuberculata has a competition with the vector species of fluke.