The size and shape of the cutting edge of the mandibles from the five most abundant copepod species found in Mejillones Bay are described with the aim to create a helpful tool for the identification of copepod prey and their size from predator gut contents. Good allometric relationships were found between the carapace length and the mandible width for the species Paracalanus parvus, Centropages brachiatus and Acartia tonsa. By contrast, the cyclopoids Oithona sp. and Corycaeus sp. did not present a good relationship between these two parameters, presumably due to the presence of more than a species in the study area. Applying the edge index (<A HREF="#itoh70">Itoh 1970</A>) the copepods were classified as herbivores (e.g., P. parvus), omnivores (e.g., C. brachiatus and A. tonsa), and carnivores (e.g., Oithona sp.). In general, there was a tight relationship between the morphometric characters of the mandible blade and the trophic ecology of each species. The good relationship between the mandible width and the carapace length of the calanoid species will permit the estimation of the size of an ingested copepod by a predator, within a certain degree of accuracy, by measuring the width of the mandibles found in gut contents. This relationship and the supplementary characterization of the mandible blade will help improve the knowledge of the feeding ecology of the mesozooplankton in northern Chile