The literature on the geographic distribution and the habitat of Cotula coronopifolia L. and Ranunculus sceleratus L. is reviewed, and observations on the population development of these species during their colonization of Zuid Flevoland, a new polder in The Netherlands, are reported. A discussion is also given of some ecological characteristics indicating that C. coronopifolia, a neophyte in Europe and introduced from southern Africa, is less adapted to its habitat than R. sceleratus, which explains the ephemeral behaviour of the former species.