The impact of the IMF on the observational properties of galaxies is perhaps most acute in the early Universe where galaxies are dominated by emission from massive stars. Using the very-large Bluetides cosmological hydrodynamical simulation we have investigated the impact of the assumed IMF on the observational properties of galaxies, including broadband photometry and nebular emission, in the early Universe. Of particular note is the sensitivity of the ionising photon production efficiency to the choice of IMF. This quantity is used to convert the observed UV emission from galaxies to an ionising photon luminosity and is thus essential when attempting to constrain the contribution of star formation to the process of reionisation.