AlGaAs nanowires are ideal waveguides for the observation of nonlinear effects. The sub-micron geometry and tight field confinement, due to the high index contrast between core and cladding, yield very intense pulsed beams at low power levels. Observations of Kerr nonlinearities are thus possible at power levels of a few watts in just a few millimeters of propagation. By using pulses of 52 W of peak power we obtain a 0.9? nonlinear phase shift from self-phase modulation. By way of cross-polarised co-propagating pulses we also obtain a crossphase modulation to self-phase modulation ratio of 0.88 at 1557 nm. Nonlinear transmission arising from three photon absorption is also analysed. We find good agreement between our experimental results and those from our simulations based on a nonlinear Schrödinger model. These results may eventually lead to the development of nonlinear all-optical devices operating in sub-micrometer interaction lengths with but a few watts of power. © 2007 IEEE.