Abstract
A ז-meson decay has been observed in an Ilford G 5 emulsion exposed under 14⋅5 cm of iron at an average height of 35000 ft. Measurements made on the secondary particles suggest that they are all
π
-mesons. The
Q
-value of the decay is (79±3⋅5) MeV, corresponding to a mass for the ז-meson of (983±10)
m
e
. The data now available on ז-mesons are briefly discussed.