This paper deals with the determination of the main factors in¯uencing series resistance in p-on-n GaAs solar cells working at concentration levels of 1000 suns or higher. Prior to this analysis, a comparison between dierent front metal grid geometries is presented to show the strong in¯uence that the front grid component of series resistance exerts on its global value. Once the inverted square grid geometry is selected, a detailed analysis of the dierent components of series resistance is carried out. For this purpose, a multidimensional optimisation of the whole GaAs solar cell (antire¯ecting coatings, series resistance and semiconductor structure) has been used for the ®rst time. In order to orient the manufacture of very high concentrator GaAs solar cells, recommendations on the threshold values of solar cell size, speci®c p-and n-contact resistances, thickness of the front metal grid and both doping level and thickness of the substrate are formulated. Several traditional ideas on the in¯uence of these parameters are questioned.