Nettles of the two species Urtica dioica and Urtica urens are commonly infested with the aphid Macrosiphum urticæ, and amid the living aphids and a collection of moulted skins dead examples are almost certain to be found. These dead aphids are almost spherical in shape and are of a brown colour, quite different from the colour of a normal aphid. They are attached to the nettle-leaf, often to the upper surface, by their ventral side, the leaf generally being discoloured at the point of attachment. If a number of these dead aphids are collected it will be found that, after a time, hymenopterous parasites, a species of the family Braconidæ, issue from the swollen body of the dead aphids. This parasite has been identified as Aphidius avence (Hal.), and the present paper deals with the life-history and habits of this species.