SPring-8 RIKEN beamline I has been designed and developed for structural biology research by the Institute of Physical and Chemical Research (RIKEN). The beamline consists of two experimental stations for protein crystallography and small-angle X-ray scattering. Both types of experiments can be carried out simultaneously, with dichromatic synchrotron radiation emitted from two coaxial undulators with vertical polarization. The branched beams are generated by a transparent diamond crystal. With synchrotron radiation, the multiple-wavelength anomalous-dispersion (MAD) method, which gives phases from a single anomalous scatterer, has been developed. Anomalous scattering contributes a small proportion of the diffraction intensity so that the accuracy of intensity data is important. The protein crystallography branch of RIKEN beamline I has been designed based on a `trichromatic concept' to optimize MAD data collection. This concept requires the quasi-simultaneous collection, by use of a `trichromator', of three intensity data sets at three different wavelengths from a single protein crystal without changing any settings. The main feature of the concept is the minimization of systematic errors in the measurement of anomalous diffraction for the MAD method. Initial commissioning of the beamline has provided three different monochromated undulator beams, which were successfully observed on the phosphor screen located at the near end of the trichromator.