The arrangement of the crystalline cellulose microfibrils in native flax fibers was studied by means of position-resolved microbeam X-ray small-angle scattering (mu SAXS) at the ESRF. The nearly perfect alignment of the microfibrils parallel to the fiber axis could for the first time be determined directly on a single fiber; a tilt angle distribution with a mean hwhm of 3.5 degrees was measured. The lateral organization (i.e., diameter and mean distance) of the microfibrils showed no significant variation across the fiber on a length scale of 4 mu m (beam size at sample position). The advantages of the mu SAXS technique are discussed by comparing it to a conventional SAXS experiment on flax fibers.