This article describes disciplinary variation in university students’ writing, as it is reflected in the use of recurrent four-word sequences. In contrast to former studies, disciplinary categories are not adopted at the outset of the assay, but rather appear from an initial assay of variation across all writers in the corpus. Variation is proposed in the form of a visual map representing levels of similarity and difference between individual writers. Emergent disciplinary groupings are then employed as the basis for a qualitative analysis of distinctive lexical bundles. Assay discloses four main disciplinary groupings. A primary distinction emerges between hard (science/technology) and soft (humanities/social sciences) subjects, with two further groupings (life sciences and commerce) being intermediate between these two. Evidence is also found of cross-group disciplines, which draw on a variety of influences, and of specific disciplines which are internally heterogeneous. A qualitative assay of bundles which are distinctive of ‘hard’ and ‘soft’ disciplines is proposed in order to characterize the discourse functions which mark these categories.