  For filling factors $\nu$ in the range between 4.16 and 4.28, we
simultaneously detect {\it two} resonances in the real diagonal microwave
conductivity of a two--dimensional electron system (2DES) at low temperature $T
\approx 35$ mK. We attribute the resonances to Wigner crystal and Bubble phases
of the 2DES in higher Landau Levels. For $\nu$ below and above this range, only
single resonances are observed. The coexistence of both phases is taken as
evidence of a first order phase transition. We estimate the transition point as
$\nu=4.22$.
