In this paper we present measurements of both the quantum- and transport mobility in two populated subbands of a Si-δ-doped GaAs structure. In these structures it is expected that ionized impurity scattering is the main scattering mechanism at low temperature. We investigated this by measuring both the transport and quantum mobility. We observe that both mobilities are independent of temperature between 1.2 and 4.2 K and find a ratio of the transport to quantum mobility of typically 2-3 in both populated subbands. Both results confirm the dominant role of ionized impurity scattering in Si-δ-doped GaAs at low temperatures.