We provide a detailed and exhaustive description of forebrain regions, nuclei, and neuronal populations which express the LIM-homeodomain transcription factor Islet1 during Xenopus development. To this end, Isl1 immunofluorescence staining was combined with other regional or neuronal markers such as Nkx2.1, Pax6, GABA, nitric oxide synthase, tyrosine hydroxylase or calretinin, and with tract tracing experiments to differentiate projection neurons from interneurons. We interpret and discuss the results with regard to the developmental origin of some previously "ambiguous" nuclei of the septum and the amygdala. Thus, Isl1 appears like a prominently expressed determinant of striatal and striatal-derived regions of the subpallium, including the central amygdala, together with the dorsal septal nucleus and the lateral septum. In the diencephalon, Isl1 is a conspicuous marker of the prethalamus, the chiasmatic regions, and the preoptic region (including important dopaminergic populations). The tuberal and mammillary parts of the hypothalamus also strongly express Isl1. From a comparative point of view, a major difference with mammals is the scarce expression of Isl1 in the embryonic medial ganglionic eminence, which is notably devoid of Isl1 expression in mammals, and the important retrochiasmatic and mammillary Isl1 expression, both also devoid of Isl1 expression in mammals. Finally, we provide evidence for the existence in Xenopus of a "new" caudal medial telencephalic nucleus, the POC (for preoptic commissural area), which was recently described in mammals.