The marsh frog (Pelophylax ridibundus sensu lato) is the number one amphibian invader in Western Europe. In Switzerland, marsh frogs were introduced in the 1950-1960s and progressively colonized most of the northern parts of the country. We investigated this invasion using molecular tools. We mapped the cryptic presence of three monophyletic mitochondrial lineages (P. ridibundus, P. kurtmuelleri and P. c. f. bedriagae from southeastern Europe) consistent with registered importations by a local frog-leg industry. High nuclear diversity supports that invasive frogs probably originated from genetically-rich import batches and patterns of population differentiation confirm that multiple independent introduction sites were involved. Moreover, several lines of evidence suggest occasional hybridization with local hybridogenetic water frogs. This invasion emphasizes the issues of frequent amphibian releases and translocations at the international and regional scale for commercial and recreational purposes, and stresses for more adequate legislation, control and information to the general public. Given the parallel invasion by alien pool frogs (i. e., the Italian P. bergeri has replaced the local P. lessonae), the situation of water frogs in Switzerland is critical. The water frog complex makes an alarming symbol of the anthropogenic mark left on wildlife diversity and distributions.