We report seven new localities of the terrestrial slug Deroceras turcicum (Simroth), including the first occurrences in the Czech Republic and Slovakia. These demonstrate that the distribution of D. turcicum extends much further north than known previously and it might even be rather common there. We discuss possible reasons for its belated discovery. D. turcicum might have often been confused not only with the common D. reticulatum (O.F. Müll.) but also with the syntopical D. rodnae Grossu et Lupu and D. praecox Wiktor. We describe its variability in size, coloration, genital morphology, and caecum length, and consider characters for its reliable discrimination from other species. Two colour morphs (white and violet) are described for the first time. Intriguingly, the coloration of D. turcicum often matches that of congeners found at the same site, which suggests that colour has some selective value. Since external appearance often gives no reason to suspect that two species co-occur at a site, dissection of several specimens is advisable.