Microstomum lineare is a common species of fresh and brackish waters found world-wide. Three genes (18S, CO1 and ITS) were sequenced from specimens of M. lineare collected from four countries, and the levels of cryptic diversity and genetic structuring were assessed. Results showed M. lineare has very wide haplotype distributions suggesting higher than expected dispersal capabilities. In addition, three new species were described on the basis of molecular taxonomy: Microstomum artoisi sp. nov., Microstomum tchaikovskyi sp. nov. and Microstomum zicklerorum sp. nov.