We present an in vivo dark-field reflection-mode photoacoustic microscopy system that performs cross-sectional (B-scan) imaging at with real-time beamforming and 3-D imaging consisting of 166 B-scan frames at with postbeamforming. To our knowledge, this speed is currently the fastest in photoacoustic imaging. A custom-designed light delivery system is integrated with a ultrasound linear array to realize dark-field reflection-mode imaging. Linear mechanical scanning of the array produces 3-D images. The system has axial, lateral, and elevational resolutions of 25, 70, and , respectively, and can image deep in scattering biological tissues. Volumetric images of subcutaneous vasculature in rats are demonstrated in vivo. Fast 3-D photoacoustic microscopy is anticipated to facilitate applications of photoacoustic imaging in biomedical studies that involve dynamics and clinical procedures that demand immediate diagnosis.