  We report the first mode-locked fiber laser to operate in the femtosecond
regime well beyond 3 {\mu}m. The laser uses dual-wavelength pumping and
non-linear polarisation rotation to produce 3.5 {\mu}m wavelength pulses with
minimum duration of 580 fs at a repetition rate of 68 MHz. The pulse energy is
3.2 nJ, corresponding to a peak power of 5.5 kW.
