The realisation of a thermo-optically controlled symmetric Mach-Zehnder interferometer switch based on an AlGaAs/GaAs epitaxial waveguide structure operating at wavelengths in the region of λ=1550 nm is reported. The device is based on a very compact two-dimensional photonic crystal channel waveguide structure. The measured and simulated transmission spectra for the devices are in good agreement. The π-phase shift switching power for the device is as low as 42 mW.