IT is generally accepted that polarized cell expansion and the strict control of division plane alignment are prerequisites for ordered spatial development in higher plants(1). This appears to be linked to the presence of cell walls, which immobilize the cells and fix their relative positions. In this context, the cortical cytoskeleton is thought to play a central role(1-6). Interphase microtubules are often aligned perpendicular to the growth axis and it has been proposed that they control cell expansion, probably in combination with the cell wall. Another cytoskeletal array, the prephophase band, has been associated with division plane alignment. This structure, which girdles the cell at the G2 phase of the cell cycle and at prophase, precisely predicts the future division site and probably fixes it. Here we describe different mutants in Arabidopsis that are unable to form these two cortical microtubular arrays. As expected, this defect is associated with irregular cell expansion and the inability to align division planes. Surprisingly, however, the mutations do not affect differentiation patterns : all cell types and organs are in their correct relative positions.