Near-single-cycle mid-infrared pulses with a spectrum covering 5.4–11 ìmare efficiently frequency-doubled in different GaSe crystals. The second-harmonic spectrum spans 3–4.3 ìm at a power conversion efficiency of >20%. We measure an effective nonlinear coefficient of deff ≈35pm∕V. We also report on self-phase modulation and spectral broadening of the mid-infrared pulses in various bulk materials and find an increase of 45% of spectral width for 5 mm of Ge. These results demonstrate that nonlinear optical conversions can efficiently be driven by few-cycle mid-infrared radiation.