In an emulsion block detector¹ exposed in the stratosphere three cases of the production ofτ-mesons in nuclear disintegrations have been observed and 5 cases of negativeK-mesons which when brought to rest are captured by nuclei. In one case the production of aτ-meson is associated with the production of a charged hyperon in the same nuclear event. In another case the production of a negativeK-meson is associated with the production of a slow positiveK-meson. The production ratio of slowτ-mesons to slow positiveπ-mesonsτ/π⁺=1·1⁺0·8% and the ratio of slow negativeK-mesons to slow negativeπ-mesonsK⁻/π⁻=0·95±0·5%. The mass of theτ-meson is found to be$$M_\tau = 975 \cdot 9 \pm 2 \cdot 2 m_e $$ and that ofK⁻-mesons$$M_{K^ - } = 927 \pm 75 m_e $$ Proof is given that all three of the decay products of theτ-mesons areπ-mesons. It is suggested that allτ-mesons observed to decay in emulsions are positively charged and that theK⁻-mesons observed by us may be the negative counterpart of this particle. Possible relations between the various phenomenologically distinguished types of chargedK-mesons are discussed.