Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). However, a phase modulator operating at the frequency-converted wavelength would be needed, which is a non-standard component.<p> </p> Fortunately, nonlinear conversion processes rely on a phase-matching condition, correlating not only the wave-vectors of the coupled waves, but also their phases. It is therefore possible to control the phase indirectly, using more standard phase modulators.<p> </p> Feasibility of this technique was previously demonstrated for second harmonic generators (SHG). Controlling the phase of the fundamental wave, excellent harmonic wave combining efficiency was achieved in both cases of phase matching and quasi phase matching, with lower than λ/30 residual phase error. <p> </p>In this paper, coherent combining of difference frequency generators (DFG) is experimentally tested. Even if DFG is more challenging than SHG as it implies handling three waves instead of two, phase control of the sole 1-μm pump waves is sufficient to combine the 3.4-μm waves generated. <p> </p>The mid-infrared DFG crystals are pumped and signal-seeded with standard all-fiber sources at 1 μm and 1.5 μm respectively. Phase control is performed with an electro-optic phase modulator which is a standard all-fiber component operating at 1 μm. <p> </p>CBC of mid-infrared DFG modules is a first step towards combining continuous wave OPOs.