The present work provides morphological informations on two zoeal stages of the alpheid shrimp Betaeus truncatus obtained from plankton samples in the southwestern Atlantic Ocean. Larvae of B. truncatus were found in coastal waters of Argentina (47°-55°S) in autumn (April) and spring (October) mainly, reaching up to 846 ind.*1000-3 per station. Larval identification of the species was confirmed by the only known previous description of the second zoeal stage of this Caridean shrimp from shallow waters of central Chile. Differences in larval size, development of the appendages and telson indicate the present larval stages to be more advanced than that of previous work which are discussed as adaptations at latitudinal changes of the environment. Since the knowledge on life history and larval morphology in the Alpheidae is extremely scarce, we provide these descriptions in order to facilitate future ecological work on meroplankton, as well as these features could give more insights into phylogenetic relationships on the basis of larval characteristics.