The aim of this work was to compare the morphometric and meristic characteristics of whitemouth croaker, Micropogonias furnieri, from two different areas: the Río de la Plata (to the west) and the oceanic coast (to the east) during the summer spawning season (December 1999), and to determine whether they constitute one demographic unit or independent management groups. Data from 966 whitemouth croakers were analysed for stock discrimination between areas. The results provided further evidence that the two cohorts differ significantly in these areas. However, an important misclassification of individuals was found. The morphometric variables that contributed more to discriminate the groups were mouth length, head length, horizontal eye diameter and length of the pectoral fin. Misclassified individuals from the western and eastern groups have shown that whitemouth croaker do not display high fidelity to their known spawning grounds. The well-classified and misclassified individuals for the western and eastern groups were found to be mature and in spawning condition, representing between 82–84%, and 66–88%, respectively. These results support the hypothesis of contemporary gene flow between the Río de la Plata and coastal Atlantic populations. Despite the misclassified individuals, morphometrics presented in this study and genetic information originating from other studies show differences between Río de la Plata and oceanic groups, and predict the two-stock hypothesis. Problems related arising from these mixed population groups and potential implications for managements are discussed.