The invasive freshwater snail Aplexa marmorata was shown to exhibit a univoltine reproductive pattern in two ecologically different habitats, a stable ornamental pond and a small, unstable lake. Generation times and the timing of egg-laying periods did, however, differ between these two sites. It is suggested that the retention of a univoltine pattern in the lake, despite a prolonged drop in water level, may be evidence that A. marmorata is not as reproductively flexible as Physa acuta, the other physid invader in South Africa. Such a lack of reproductive flexibility may explain why A. marmorata has not invaded rivers whereas P. acuta, which is known to adopt a multivoltine pattern following repeated disturbances to its habitat, has done so.