Synthetic interspecific hybrids and backcross progeny of Ranunculus flammula L. and R. reptans L. have been compared for their morphology, phenotypic response to submersion, fertility, and breeding system, with transplant specimens of R. reptans-like plants from Loch Leven. The two species were found to be fully inter-fertile. The Loch Leven transplants were found to show a striking similarity to backcross progeny of F1 interspecific hybrids x R. reptans, thus providing experimental evidence for the introgressive hybrid status of these plants as proposed by previous authors.