Abstract: A disposable maximum reading thermometer comprises a body (40) in which a capillary passage is defined, a reservoir (41) communicating with the passage and a heat expansible indicator medium (46) contained in the reservoir. The reservoir and the passage are adapted to allow expansion of the medium in the passage when the medium is subjected to heating. At least a part of the passage wall exposable to the indicator medium is adapted to undergo a detectable and irreversible change of its light penetration characteristics when brought into contact with the medium. The capillary passage is defined by a rod (48) arranged in a longitudinally extending bore (42) of the body (40). The surface of the rod is frosted (matt) or opalized, preferably by a mechanical treatment, and adapted to be deopalized when brought into contact with the medium.