Fluorescent oligonucleotide probe having a general formula (I), wherein Onu represents an oligonucleotide residue, Thio.sub.n represents a fluorescent oligothiophene containing n thiophenic rings, n being an integer lower than 8, L representing a binder conceived to maintain Thio.sub.n mobile in relation to Onu in a manner so that Thio.sub.n can perform its fluorescent action correctly, and Onu can perform its hybridization action. (I)=Onu-L-Thio.sub.n.