The invention relates to novel fluorescent rhodamine dyes with enhanced cell permeability which are rhodamine 4′-isomers having the following general structural formula A: wherein Z is selected from O(alkyl), O(aryl), S(aryl), S(O)(alkyl), S(O)(aryl), S(O)2(alkyl), S(O)2(aryl), S(O)2(-O-alkyl), S(O)2(-O-aryl), S(O)2NH(alkyl), S(O)2NH(aryl), S(O)2N(alkyl)2, S(O)2N(aryl)2, S(O)2N(alkyl)(aryl), C(O)O(alkyl), C(O)O(aryl), C(O)(alkyl), C(O)(aryl), P(O)OH(-NH-alkyl), P(O)OH(-O-alkyl), P(O)OH(-NH-aryl), P(O)OH(-O- aryl), P(O)(-O-alkyl)2, P(O)(-NH-alkyl)2, P(O)OH(-N(alkyl)2), P(O)OH(-N(aryl)2), P(O)(-N(aryl)2)2, P(O) (-N(alkyl)2)(-N(alkyl)2), P(O)(-O-aryl)2, P(O)(-NH-aryl)2, P(O)(-O-alkyl)(-O-aryl), P(O)(-NH-alkyl)(-O-aryl), P(O)(- O-alkyl)(-NH-aryl), P(O)(-NH-alkyl)(-NH-aryl), C(O)OH, C(O)NH(alkyl), C(O)NH(aryl), CON(alkyl)2, CON(aryl)2, in particular C(O)OH, C(O)NH(alkyl), C(O)NH(aryl), CON(alkyl)2, CON(aryl)2, C(O)O(alkyl) and C(O)O(aryl), or any group which is neither Cl, NH2 or NO2 and which induces a neighboring group effect via steric, ionic or bonding interactions with the adjacent carboxyl group resulting in a shift of the equilibrium between zwitterionic form and spirolactone form towards the spirolactone form. The invention further relates to 4′-isomer derivatives and probes comprising such 4′-isomers coupled to at least one reactive group or ligand which is capable to interact with or bind to other molecules, wherein said reactive group or ligand may be coupled to the rhodamine 4′-isomer fluorophore either directly or via a linker. Another aspect of the invention relates to the use of these compounds and conjugates as labels in microscopic, spectroscopic and other imaging techniques and/or as cell permeable substances penetrating through membranes of living and fixed cells in vivo or in vitro.