  Muon abundance is required for the understanding of several fundamental
questions regarding properties of the primordial Universe. In this paper we
evaluate the production and decay rates of muons in the cosmic plasma as a
function of temperature. This allows us to determine when exactly the muon
abundance disappears. When the Universe cools below the temperature
$kT_\mathrm{disappear}\approx 4.135$ MeV the muon decay rate overwhelms
production rates and muons vanish quasi-instantaneously from the Universe.
Interestingly, we show that at $T_\mathrm{disappear}$ the muon number is nearly
equal to baryon abundance.
