This paper is an account of the anatomy of the larval and adult stages of the sawfly Perga affinis affinis Kirby. Particular attention has been paid to those structures that seem relevant to the ecology of the animal. In this respect, this work is supplementary to that of Carne (1962) on the ecology of this insect. The external features of the larva and adult are described. Measurements of head capsules of the larvae showed that these could be employed as a method of identifying the various instars. From observations made by Carne (op. cit.) it is probable that the species breeds parthenogenetically. However, the male reproductive system appears quite functional. The structure and musculature of the ovipositor is described, as well as the internal reproductive organs. The muscles of the thorax are compared with those described by Snodgrass (1927) in the generalized insect and by Tiegs (1955) in the various orders. In P. affinis affinis the prothoracic muscles are modified for the movement of the head and prepectus. The development of flight muscles is greatest in the mesothorax where the indirect flight muscles almost completely fill the cavity. The indirect flight muscles of the metathorax are poorly developed. The muscles are composed of numerous fibres. There has been no reduction and enlargement of the fibres as in the Diptera. The coxal muscles have become separated from the flight muscles and are confined to the sterna and furcae.