Fused pyridinium thiol/sulfide compounds (A) and their organic or mineral acid salts, optical isomers, geometrical isomers, solvates or hydrates, useful as fluorescent dye, are new. Fused pyridinium thiol/sulfide compounds (A) of formula (I) or (II) and their organic or mineral acid salts, optical isomers, geometrical isomers, solvates or hydrates, useful as fluorescent dye, are new. Ring A : aryl, heterocycle, heteroaryl fused with phenyl ring or absent G, G1a : -NR cR dor 1-6C alkoxy (optionally substituted) either R c, R dH, 1-6C alkyl (optionally substituted), aryl(1-4C)alkyl or (1-6C)alkoxy(1-6C)alkyl or NR cR dheterocycle or heteroaryl either R g, R1g, R2g, R3g, R h, R1h, R2h, R3h : H (preferred), halo, amino, (di)(1-4C)alkylamino, CN, carboxy, OH, CF 3, acylamino, 1-4C alkoxy, 2-4C (poly)hydroxyalkoxy, (1-4C)alkylcarbonyloxy, (1-4C)alkoxycarbonyl, (1-4C)alkylcarbonyl-amino, acylamino, carbamoyl, (1-4C)alkylsulfonylamino, amino-sulfonyl or 1-16C alkyl (optionally substituted by 1-12C alkoxy, OH, CN, carboxy, amino or (di)(1-4C)alkylamino), where the two alkyl together with nitrogen of the group amino form 5-7 membered heterocycle (optionally containing another heteroatom same or different to the one of the nitrogen atom) or R gR1g, R2gR3g together with two atoms of adjacent C : benzo ring, indeno ring, fused heterocycloalkyl or fused heteroaryl (all optionally substituted by halo, 1-4C alkyl, amino, (1-4C) alkylamino, (1-4C)dialkylamino, CN, carboxy, OH, CF 3, acylamino, 1-4C alkoxy, 2-4C (poly)hydroxyalkoxy, alkylcarbonyloxy, alkoxycarbonyl, alkylcarbonylamino, acylamino, carbamoyl, alkylsulfonylamino, amino-sulfonyl or 1-16C alkyl (optionally substituted by 1-12C alkoxy, OH, CN, carboxy, amino, 1-4C alkylamino or 1-4C dialkylamino, where the two alkyl together with nitrogen of the group amino form 5-7 membered heterocycle (optionally containing another heteroatom same or different to the one of the nitrogen atom))) or R cR1g, R dR g, R cR2g, R dR3g (when G,
