We report the first (to our knowledge) experimental results and numerical simulations on mid-IR femtosecond pulse filamentation in argon using 0.1 TW peak-power, 80 fs, 3.9 μm pulses. A broadband supercontinuum spanning the spectral range from 350 nm to 5 μmis generated, whereby about 4% of the mid-IR pulse energy is converted into the 350–1700 nm spectral region. These mid-IR–visible coherent continua offer a new, unique tool for time-resolved spectroscopy based on a mid-IR filamentation laser source.