MAP65 comprises a multigene family specific to plants. To see which isoform is utilised for the unique mechanism of cell expansion, uncomplicated by division structures, carrot cells were deprived of auxin whereupon they stopped dividing and elongated instead. During elongation, a MAP65 protein triplet reduced to a single band. Mass spectrometric analysis demonstrated that this corresponded to a single carrot cDNA; it also corresponded to the major protein previously shown to form filamentous cross-bridges between microtubules in vitro. This MAP65 isoform is concluded to have a major role in establishing the parallel microtubule arrays characteristic of cells undergoing directional expansion.