The life cycle and structure of the larva of Austramphilina elongata using light-microscopy, scanning and transmission electron microscopy are described. Eggs are round and non-operculate. Larvae hatch in freshwater and penetrate through the cuticle of juvenile crayfish, Cherax destructor, and of freshwater shrimps, Paratya australiensis and Atya (= Atyoida) sp., shedding their ciliated epidermis. In the last two hosts, development to the infective stage does not occur. In crayfish, larvae grow and reach the infective stage. Turtles, Chelodina longicollis, become infected by eating infected crayfish. Larvae penetrate through the oesophageal wall of the turtle and migrate toward the coelom, where maturation occurs. The free-swimming larva has a syncytial epidermis which covers most of the body except for the posterior region bearing the hooks. It is loosely attached to a thin underlying tegument, which is connected to ‘insunk’ nucleated cell bodies. It forms a thick surface layer in the posterior region. There are three flame cells on each side of the body and two postero-lateral excretory pores. There are no lateral flames. The weir apparatus of the flame cell has the structure typical of parasitic platyhelminths. The smaller capillaries have a smooth surface, that of the terminal ducts is covered by numerous microvilli. Three types of penetration glands open anteriorly. There are five pairs of hooks; one median ‘normal’, two submedian halberd-shaped, and two lateral serrate. Hook are not lost, they are arranged around the gonopore of the adult. Frontal glands opening into the proboscis were found in the anterior part of the body in all stages examined. Infective stages in crayfish have developing reproductive organs and ducts. The tegument of the adult has many microvilli.