A device for determining at least one parameter of a medium flowing in a line, particularly the intake air mass of an internal combustion engine, having one part which has at least one measuring channel for conducting at least one partial flow of the medium flowing in the line in a main flow direction and which can be inserted into the line with a predetermined alignment with respect to the main flow direction, and having at least one measuring element situated in the measuring channel for determining the at least one parameter. In the part, a channel structure is formed, having an input region for the entry of a partial flow of the medium and having a measuring channel branching off from the input region. The input region has a separation zone with a separation opening, and at least two projections which protrude from mutually opposite interior walls of the input region into the input region.