The construction and operation of a 100 MHz B-mode ultrasound backscatter microscope are described. The powerful B-mode technique is extended into the domain of microscopy allowing the imaging of internal structure in living specimens on a microscopic scale. A frame rate of 5 frames per second is achieved which gives rapid feedback to the operator. Specially designed components of the scanner are described in detail, including the transducer, motion system and scan converter. An f/2 transducer is employed, leading to a scanner resolution of approximately 36 micron in both the lateral and axial directions. The benefits of such high resolution are demonstrated in preliminary images of multicellular spheroids and intact human ocular tissue.