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Background. Although the species of the family Gerreidae have been subjected to many studies, their taxonomy at specific- and sometimes generic levels is still confusing. To contribute to the clarification of the taxonomy of the Mexican Pacific mojarras, the morphology of sagittal otoliths among six species of mojarras (Diapterus brevirostris, Eugerres lineatus, Eucinostomus dowii, E entomelas, E. currani, and Gerres cinereus) was compared using size and shape descriptors. Otolith shape has long been known to be species-specific but this has not been tested in species of the family Gerreidae. Therefore, our goal was to explore the effectiveness of otolith descriptors for identifying gerreid fish at species level. Materials and methods. Gerreid fish were captured between January 2009 and January 2010, off the Pacific coast of Mexico. The right- and left sagittae of 160 individuals were extracted for analysis. Size and shape descriptors considered in the presently reported analysis were: surface area, perimeter, length, width, rectangularity, ellipticity, roundness, circularity, aspect ratio, form-factor, Feret length, Feret width, Feret maximum, Feret minimum, diameter maximum, diameter minimum, diameter mean, ratio maximum, and ratio minimum. Other measurements were made in the otolith region of sulcus acusticus, such as: cauda length, ostium length, ostium width, sulcus length, and rostrum width. Canonical discriminant analysis on otoliths morphology was used to identify differences between species. Results. Rectangularity, roundness, otolith length, and Feret length were the main otolith descriptors that explain the inter-specific variability. Significant differences (Wilks' lambdas (λ), P < 0.001), high canonical correlation coefficients, and also a high classification success (overall mean >90%) allowed the separation of the species by using discriminant functions. Results from both G-test and Cohen's kappa procedure confirmed the high rates of classification success obtained by the discriminant analysis. Conclusion. These results suggest the usefulness of otolith morphology for differentiation of Gerreidae species from Mexican Pacific waters, thereby demonstrating that otolith shape is species-specific. Otolith morphology descriptions provided in this study is presented for the first time for the species included. [ABSTRACT FROM AUTHOR]