The effect of the ultrasonic output of a commercial cat 'deterrent' was assessed by measuring the behaviour responses of 10 cats in a standard test arena. The cats were introduced to the arena approximately nine metres outside the stated range of efficacy of the product and small food piles were placed at one metre intervals towards the device. When the cats were released from the basket, their behaviour and location were recorded continuously. The behaviour of the cats with the device on and off was compared by using a general linear model and chi-squared analysis. Differences between individual cats were a significant factor in explaining the variance associated with the amount of 'relaxed behaviours' (P<0.001), and the time spent within the range of the device (P=0.006). The only significant behaviour changes recorded when the device was on, were an increased likelihood of ear flicking (P<0.001), less time spent actively exploring (P=0.043), and an increase in the amount of time spent in the reported ultrasonic range of the device (P=0.003).