This paper presents a resonator-fed dual-frequency dual-mode bidirectional leaky-wave antenna to provide attractive high-gain broadside radiation at dual desired frequencies. First, by analyzing the dispersion diagrams of the first (EH1) and third (EH3) higher order modes of a microstrip line, it is theoretically verified that these two modes can both be suitably employed for bidirectional leaky-wave antenna design. Additionally, it is also determined that the ratio between these two operation frequencies can be handily controlled by appropriately loading periodical shorting pins along the nodal lines of the EH3 modal field. As such, the dual-frequency operation principle is thoroughly investigated and determined. Afterward, to effectively excite the desired modes and suppress any unwanted modes of the microstrip line, a dual-frequency resonant-fed strategy is adopted. More specifically, the introduced feeding resonator is first fed by a pair of differential probes, and then the microstrip EH1 and EH3 modes are excited through the coupling gap. In this way, the desired pure EH1 and EH3 modes are successfully excited. As an example, a dual-frequency bidirectional microstrip leaky-wave antenna is designed to work at 3.6 GHz and 4.9 GHz, and its prototype is simulated, fabricated, and tested. The obtained far-field radiation patterns from simulations and measurements show a reasonable agreement, and the concerned high-gain radiation at broadside is well achieved at both frequencies.