  We report on muon-spin rotation/relaxation (\muSR), electrical resistivity,
magnetization and differential scanning calorimetry measurements performed on a
high-quality single crystal of Cs_0.8(FeSe_0.98)_2. Whereas our transport and
magnetization data confirm the bulk character of the superconducting state
below T_c = 28.5(2)K, the \muSR data indicate that the system is {\it magnetic}
below T_N = 478.5(3)K, where a first-order transition occurs. The first-order
character of the magnetic transition is confirmed by differential scanning
calorimetry data. Taken all together, these data indicate in
Cs_0.8(FeSe_0.98)_2 a microscopic coexistence between the superconducting phase
and a strong magnetic phase. The observed ratio between T_N and T_c is the
highest one reported to date for a magnetic superconductor.
