NOVELTY - A polyarylene containing hydrophobic and hydrophilic units (I) is new.    USE - As a polymer used in polymer electrolyte membrane, membrane-electrode assembly, fuel cell system, and as binder (claimed).    ADVANTAGE - The polymer has both hydrophobic and hydrophilic properties and weight average molecular weight of 1000-1000000 (preferably 20000-500000), thus has excellent proton conductivity or ion conductivity, shape stability, and long-time stability, high proton conductivity at a low humidifying condition and excellent mechanical properties, and prevents cross-over of fuel.    DETAILED DESCRIPTION - A polyarylene containing hydrophobic and hydrophilic units of formula -((A1)l1-(Ar)m1((R1)n1)-A2-(Ar)m2((R2)n2)-(A3)l2-(Ar)m3((R3)n3)-B1-X-B2-(Ar)m4((R4)n4))p-((A4)l3-(Ar)m5((R5)n5)-A5-(Ar)m6((R6)n6)-(A6)l4-(Ar)m7(Y 1)-B3-(Ar)m8(Y2))q- (I) is new.    X=a cycloalkylene or is of formula -(Ar)m9((R9)n7)-(A7)l5-(C(R11)(R12))m11-(A8)l6-(Ar(R10)n8)m10 (II);    A1, A3, A4, A6, A7 and A8=CR7R8, O, S, CO, SO, and SO2;    A2 and A5=alkylene, cycloalkylene, fluorenylene, arylene (all optionally substituted) or CR7R8;    Ar=an arylene group;    B1-B3=O, S, CO, SO, or SO2;    R1-R10=alkyl, cycloalkyl, alkenyl, alkoxy, (hetero)aryl, or heteroalkyl including an atom selected from O, N, S (all optionally substituted) or H;    R11 and R12=alkyl, alkenyl, alkoxy, (hetero)aryl, or heteroalkyl including an atom selected from the group consisting of O, N, or S (all optionally substituted), H, or fluorine;    Y1 and Y2=a proton conductive group;    m1-m10=0-10;    l1-l6=0-30;    m11=1-40;    p=1-10000;    q=1-10000;    n1-n8=1-20.    Provided that:    (1) at least one of m1-m4 is not 0, at least one of m5 and m6 is not 0, at least one of m7 and m8 is not 0, and at least one of m9 and m10 is not 0; and    (2) at least one of l1 and l2 is not 0, at least one of l3 and l4 is not 0, and at least one of l5 and l6 is not 0.    INDEPENDENT CLAIMS are included for the following:    (1) a fuel cell having a polymer electrolyte membrane comprised of the polymer (I);    (2) a fuel cell having a catalyst layer including a binder comprised of the polymer (I);    (3) a membrane-electrode assembly (17) for a fuel cell comprising a cathode and an anode facing each other, each of the cathode and the anode comprising an electrode substrate and a catalyst layer comprised of a catalyst and a binder; and a polymer electrolyte membrane interposed between the cathode and the anode, at least one of the binder and the polymer electrolyte membrane comprising a polymer (I); and    (4) a fuel cell system (1) comprising at least one electricity generating element (3), each electricity generating element comprising: a membrane-electrode assembly and separators (19,19') arranged at each side of the membrane-electrode assembly; a fuel supplier (5) for supplying a fuel to the electricity generating element; and an oxidant supplier (7) for supplying an oxidant to the electricity generating element.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a fuel cell system.    Fuel cell system (1)    Electricity generating element (3)    Fuel supplier (5)    Oxidant supplier (7)    Separators (19,19')