This paper details the embryonic development of the triclad flatworm Schmidtea mediterranea and describes, for the first time in flatworms, the phenomenon of adelphophagy. Embryonic development, which takes 23 days at 18–20°C, basically corresponds with the developmental stages documented in other studies for triclad flatworms. At some of the stages it was observed occasionally that some embryos cannibalized their siblings, i.e. exhibited adelphophagy or embryophagy. It is suggested that this phenomenon may contribute to the fact that the number of young hatching from a cocoon is usually considerably less than the number of egg cells initially assembled in the capsule. The occurrence of adelphophagy in the animal kingdom is discussed.