A fuel cell cathode flow field has multiple channels each with a cross-sectional area that varies along the length of the channel such that oxygen availability at every lengthwise position along the channel is kept substantially constant for a given channel length and air stoichiometry ratio. Each channel comprises a flat floor with substantially constant depth and a pair of side walls extending upwardly from the floor; the side walls each taper inwards from channel inlet to outlet with a convex curve relative to the channel centreline. Achieving substantially uniform oxygen availability throughout the flow field results in substantially uniform current density throughout the flow field, which is desirable for efficient fuel cell operation and improved performance.