The nonlinear properties of microbubble contrast agents have been used to create contrast-specific imaging modalities such as harmonic imaging and subharmonic imaging. Thus, a better understanding of the nonlinear performance of contrast microbubbles may enhance the diagnostic capabilities of medical ultrasound (US) imaging. The first and second harmonic, the 1/2 order subharmonic and the 3/2 order ultraharmonic components in spectra of scattered signals from Optison microbubbles insonified at 2 and 4 MHz have been investigated using an in vitro laboratory pulse-echo system. The development of these signal components over time is quite different for 2-MHz insonification compared to 4-MHz insonification. Scattered subharmonic and ultraharmonic signals are much more time-dependent than first and second harmonic echoes. The dependence of the first and second harmonic, subharmonic and ultraharmonic components on acoustic pressure for 2-MHz insonification is similar to that for 4-MHz insonification. The first and second harmonic components increase linearly with acoustic pressure (in double logarithmic scales) and the subharmonic and ultraharmonic amplitudes undergo rapid growths in the intermediate acoustic pressure range and much slower increases at both lower and higher acoustic pressures.