  We have detected the 372 GHz line of ortho-H2D+ towards the pre-stellar core
L1544. The strongest emission (T_mb ~ 1 K) occurs at the peak of the millimeter
continuum emission, while measurements at offset positions indicate that H2D+
is confined within ~ 20 arcsec, where CO is highly depleted. The derived H2D+
abundance of ~ 10^{-9} is comparable with previous estimates of the electron
abundance in the core, which suggests that H2D+ is the main molecular ion in
the central 20 arcsec (2800 AU) of L1544. This confirms the expectations that
H2D+ is dramatically enhanced in gas depleted of molecules other than H2. The
measured abundance even exceeds the present model predictions by about a factor
ten. One possibility is that all CNO-bearing neutral species, including atomic
oxygen, are almost completely (> 98%) frozen within a radius of ~2800 AU.
