Fresh-water planarians are generally dichogamous hermaphrodites; self fertilisation has been found till now only in Cura foremanii as a normal reproductive mechanism (Anderson 1952 and Anderson & Johann 1958) and in Polycelis nigra, where it occurs only sporadically (Benazzi 1952). The Author has obtained a specimen of P. nigra, born from an individual irradiated with 4000r of X-rays, which in its second year of life has laid cocoons all of which were self-fertilised; from these a very high percentage of live specimens were hatched.