RIKEN beamline I (BL45XU) is an undulator beamline with two branches. One is for protein crystallography (PX) and the other is for small-angle x-ray scattering (SAXS). The beam is split into the two branches by a diamond monochromator so that two experiments can be done simultaneously [Yamamoto et al. (1995) Rev. Sci. Instrum. 66, 1833-1835]. The SAXS branch was designed for studying the weak interaction of proteins or subunits of fibrous or protein solutions especially using hydrostatic pressure. The optics makes use of the good parallelism of the undulator beam in order to reduce parasitic scattering. The beamline consists of a double crystal monochromator and a K-B type focusing mirror system. In order to cope with the high flux of the beam, an x-ray image intensifier (Hamamatsu Photonics, V5445P) with a cooled CCD camera (C4880-82) was used. As a result, decreases in both collection time and sample amount were realized in standard static experiments. These improvements will greatly facilitate SAXS experiments under high pressure.