Two types of triclad in the Dugesia lugubris species complex were identified on the basis of head shape and cannibalistic behaviour. More detailed examination of the reproductive organs, breeding and cytology showed them to be distinct species. One of them fitted the description of Dugesia lugubris (sensu Schmidt, 1860), the other fitted D. polychroa (sensu Schmidt, 1862). They are most reliably distinguished by the features of the penial papilla; in D. polychroa it is relatively small, with a ventral duct and lacking a permanent nipple, and in D. lugubris the papilla is massive, with an ejaculatory duct surrounded by thick musculature and possessing a permanent nipple. There are also differences in external appearance. The head of D. lugubris is pointed, that of D. polychroa rounded. The eyes are nearer the anterior border of the head in the former. Dugesia lugubris has a less pronounced neck and a more squat appearance than D. polychroa and is often larger. The colour and pattern of pigment is more variable in D. polychroa. The ventral surface is lighter in colour than the dorsal in D. polychroa, but in D. lugubris they are similar. It is suggested that the earlier confusion in the nomenclature of these two species was due to the failure to recognize that D. polychroa may exhibit a temporary nipple on the penial papilla. A survey of lakes in northern Britain shows that Dugesia polychroa is more common than D. lugubris. The karyotype of D. polychroa corresponds to Benazzi's type B, while that of D. lugubris belongs to type E. Most of the earlier records and literature for British Dugesia refer to D. polychroa.