This invention relates to a microstrip 20 to slotline 31 transition circuit operating on two frequency bands, in which a first filtering circuit 32 is connected between a first portion of microstrip line and a first input/output port P10, said first circuit and the portion of microstrip line being adapted to accept the frequencies from a first frequency band and reject the frequencies of a second frequency band, a second filtering circuit 33 is connected between a second portion of microstrip line and a second input/output port P20, said second circuit and the portion of microstrip line being adapted to accept the frequencies from the second frequency band and reject the frequencies from the first frequency band, and the slotline is sized to provide an impedance that is substantially equal to the impedance of an open circuit at the coupling zone between the microstrip line and the slotline.