A method for ascertaining a billing tariff for billing for a packet switched data transfer, where a mobile communication terminal which is involved in the data transfer and has access rights for a data mobile radio network is situated in a visitor mobile radio network. The method involves a data link being set up between a billing system, which performs billing for the data transfer, and a central subscriber register in the data mobile radio network, and this data link being used to transfer to the billing system an item of location information which relates to the current location of the communication terminal. The billing system uses the item of location information to establish that the communication terminal is situated in the visitor mobile radio network, and the billing system ascertains a billing tariff which is associated with the visitor mobile radio network.