In this work, two new species of the genus Macrostomum (M. velastylum n. sp. and M. puntapiedrensis n. sp.) are described from the littoral benthos of the Río de la Plata estuary, Argentina. This is the first description of members of the Macrostomidae from Argentina, and of a species documented from both limnic and brackish-water environments. Macrostomum velastylum differs from its congeners in the fine details of its stylet, which are described using a new technique for stylet isolation and observation (scanning electron microscope). This method provides new information on stylet structure relative to traditional whole-mount techniques. Macrostomum puntapiedrensis differs from others species of the M. orthostylum group in the length and morphology of the distal tip of the stylet. Further details are provided on additional macrostomids from Argentina, including potential conspecifics of M. vejdovskyi, M. viride, and M. lineare.