Ultrasound microscopes have the potential for imaging structure at depth in thick specimens, yet this is not possible in biological specimens using conventional ultrasound transmission or reflection methods. But, subsurfacing imaging is possible with ultrasound if a backscatter (pulse-echo) technique, similar to that used in medical imaging, is used. The central problem of extending backscatter imaging to ultrasound microscopy has been the development of high frequency (greater than 100 MHz) transducers with sufficient bandwidth and sensitivity to detect the low levels of backscatter from biological materials. We recently reported the development of such a transducer which we have now incorporated into a new ultrasound backscatter microscope capable of providing tomographic images at depths of up to 4 mm in biological specimens. Here we present the first ultrasound backscatter micrographs of living biological specimens. The benefits of this technique are demonstrated by its application to imaging the internal structures of living tumour spheroids showing striking contrast between the necrotic core and the viable rim of the spheroid.