Although research on sonocrystallisation of lactose has been reported in the literature (yield and crystal size), the effect of ultrasound variables on nucleation and growth rate of lactose have not been studied. In this study, lactose crystallisation with ultrasound was compared with mechanical agitation using the induction time method at 22°C. Ultrasound had a significant effect in reducing induction times and narrowing the metastable zone width but had no effect on individual crystal growth rate or morphology. A rapid decrease in induction time was observed up to 0.46Wg-1 power density. Sonication up to 3min decreased the induction time, but no further reduction was observed beyond 3min. It was not possible to generate the nucleation rates achieved by sonication using agitation alone. 1min sonication at 0.46Wg-1 power density followed by continuous stirring was found to be the optimum under the experimental conditions tested.