The invention relates to a molecular beacon in the form of a hairpin oligonucleotide or oligonucleotide analogue comprising a first nucleotide sequence containing two or more aromatic or heteroaromatic ring systems P able to form an excimer or exciplex; a second sequence (the loop) consisting of an oligonucleotide probe able to hybridise with a target polynucleotide; and a third sequence containing one or more aromatic or heteroaromatic ring systems X, wherein at least one aromatic ring system X interacts with two aromatic ring systems P of the first sequence inhibiting excimer or exciplex formation. The invention further relates to a method for detecting the presence of a target polynucleotide using such a molecular beacon, and to a kit comprising a molecular beacon of the invention for use in this method.