  We measure the resistivity and magnetic susceptibility in the temperature
range from 5 K to 600 K for the single crystals $A$Fe$_{2-y}$Se$_2$ ($A$ =
K$_{0.8}$, Rb$_{0.8}$, Cs$_{0.8}$, Tl$_{0.5}$K$_{0.3}$ and
Tl$_{0.4}$Rb$_{0.4}$). A sharp superconducting transition is observed in low
temperature resistivity and susceptibility, and susceptibility shows 100%
Meissner volume fraction for all crystals, while an antiferromagnetic
transition is observed in susceptibility at Neel temperature ($T_N$) as high as
500 K to 540 K depending on A. It indicates the coexistence of
superconductivity and antiferromagnetism. A sharp increase in resistivity
arises from the structural transition due to Fe vacancy ordering at the
temperature slightly higher than $T_{\rm N}$. Occurrence of superconductivity
in an antiferromagnetic ordered state with so high $T_{\rm N}$ may suggest new
physics in this type of unconventional superconductors.
