In this paper I propose a reading of Plotinus Enneads VI.1-3 [41-43] On the genera of being which regards this treatise as a coherent whole in which Aristotle's "Categories" is explored in a way that turns it into a decisive contribution to Plotinus' Platonic ontology. In addition, I claim that Porphyry's Isagoge and commentaries on the "Categories" start by adopting Plotinus' point of view, including his notion of genus, and proceed by explaining its consequences for a more detailed reading of the "Categories." After Plotinus' integration of the "Categories" into the Platonic frame of thought Porphyry saw the possibilities of exploiting the Peripatetic tradition both as a means to support the Platonic interpretation of the "Categories" and as a source for solutions to traditional questions. His allegiance to a division of being into ten, and his emphasis on semantics rather than ontology can be explained from this orientation. In the light of our investigation the alleged disagreement between Plotinus and Porphyry on the "Categories" changes its appearance completely. There are differences, but these can be best explained as confirmation and extension of Plotinus' perspective on the "Categories" and its role in Platonism.