1. The width and length of eggs of Haemonchus contortus, Cooperia curticei, Trichostrongylus axei, T. vitrinus, Bunostomum trigonocephalum, Chabertia ovina, Oesophagostomum venulosum and Ostertagia spp. were measured.
2. Frequency distributions of the linear dimensions of the eggs of each species are given, together with the frequencies calculated on the assumption that the distributions are normal.
3. A method of species differentiation based on egg measurements is described, the method being based upon the statistical chances of eggs of different species falling into particular size classes.
4. The application of the method and the construction of a classifying scale are described.