Dynamic nonlinearity can manifest as changes in characteristic properties of a vibrating system in response to variations in excitation. This study investigates the nonlinearity in the vibration response of the flexural ultrasonic transducer. This device is typically employed for industrial measurement, but little is known about the influence of changes in excitation on its dynamics. In general, the resonance frequency of an ultrasonic device is known to shift as excitation amplitude is increased, displaying either hardening nonlinear behaviour, where resonance frequency increases, or softening associated with resonance frequency decrease. In typical operation, the vibration response of the flexural ultrasonic transducer has been found to be weakly nonlinear. Different physical mechanisms can cause nonlinearity, including structural configuration, the physical responses of components such as the transducer membrane, and thermomechanical properties inherent in piezoelectric materials. The nonlinear behaviour of flexural ultrasonic transducers is shown in the context of typical operation in practical application, through laser Doppler vibrometry and supported by fundamental mathematics. The results demonstrate the existence of nonlinear behaviour for different types of flexural ultrasonic transducer for modest changes in excitation amplitude, and show that the influence of dynamic nonlinearity should be considered in the practical application of flexural ultrasonic transducers.\ud