This paper examines the development of the oral morphology of the parasite Salmonema emphemeridarum (Nematoda: Cystidicoidae) using Scanning Electron Microscopy. Larvated eggs, taken from female worms collected from brook trout, Salvelinus fontinalis, were fed to mayflies (Ephemeroptera) where larvae developed to the third larval stage. First stage larvae possessed an oral opening, boring tooth and secretory pore. Second stage larvae possessed a circular oral opening and amphids. Third stage larvae included all the features of adult worms include pseudolabia, submediant labia, sublabia, amphids and four oral papillae. The advanced development of oral structures of third stage larvae allows the identification of larval worms to genus and in some cases species and is consistent with the precocious reproductive development of infective larvae in the Cystidicolidae.