We report observation of the 1S-2S two-photon transition in hydrogen c
onfined in a magnetic trap at submillikelvin temperatures. The excitat
ion spectrum can display a sharp central feature arising from periodic
motion of the trapped atoms. The metastable 2S atoms remain trapped,
and have a lifetime close to the natural lifetime of 122 ms. These dev
elopments open the way to achieving the transition's natural linewidth
. We have also demonstrated that the temperature of the gas can be det
ermined from the line shape, providing an important tool for the study
of cold trapped hydrogen. The resolution in these experiments appears
to be limited by laser stability, currently 3 kHz.