Morphological changes in the surface membranes of cultured Balb/c 3T3, clone 1-13 cells were studied by scanning electron microscopy after the cells were exposed to diagnostic ultrasound. Abundant irregular, dense microvilli appeared in transformed clones as well as in cells that had been exposed to ultrasound, but had not been morphologically transformed. This increased density of microvilli was detected as early as three days after ultrasound and was still present 37 days later. In comparison, unexposed control cells were smooth with occasional microvilli. Further studies are needed of the effects of diagnostic ultrasound.