This article identifies and describes the rhetorical organization of the Textbook genre, based on a corpus-based approach and on part of PUCV-2006 Corpus of Academic Spanish. This subcorpus includes 126 textbooks collected from four scientific disciplines at undergraduate university programmes: Social Work, Psychology, Construction Engineering and Industrial Chemistry. In order to fulfil the goals, the article determines and describes the communicative purposes of each of the moves and steps identified, and gives examples from the four disciplinary domains. A new macro-level to fully account for the analysis is introduced and justified, which turned out to be essential for a better description of an extensive textual unit as these texts are (200,000 words). This is named as macro-move. The main findings reveal a well-organized organization directed towards educational instruction. The genre fundamental nucleus is formed by the macro-move Conceptualization and Exercising, which is repeated as many times as the contents are displayed. By the end of the article, the question appearing in the title is answered and arguments are offered to explain why the genre's rhetorical organization is described as a 'colony-in-loops'.