  We report the first direct excitation of the strongly forbidden 5s2 1S0-5s5p
3P0 transition in 87Sr. Its frequency is 429 228 004 235 (20) kHz. A resonant
laser creates a small leak in a magneto-optical trap (MOT): atoms build up to
the metastable 3P0 state and escape the trapping process, leading to a
detectable decrease in the MOT fluorescence. This line has a natural width of
10^{-3} Hz and can be used for a new generation of optical frequency standards
using atoms trapped in a light shift free dipole trap.
