  We have conducted the first extensive observational survey of several
deuterated species in 16 Class 0/I proto-brown dwarfs (proto-BDs) and 4 Class
Flat/Class II brown dwarfs. Observations were obtained with the IRAM 30m
telescope in the DCO$^{+}$ (3-2), DCN (3-2), DNC (3-2), and N$_{2}$D$^{+}$
(3-2) lines. The DCO$^{+}$/H$^{13}$CO$^{+}$, DCN/H$^{13}$CN, and DNC/HN$^{13}$C
ratios are comparatively higher and show a narrower range than the
DCO$^{+}$/HCO$^{+}$, DCN/HCN, and DNC/HNC ratios, respectively. The mean D/H
ratios for the proto-BDs derived from these molecules are in the range of
$\sim$0.02-3. Both low-temperature gas-phase ion-molecule deuteron transfer and
grain surface reactions are required to explain the enhanced deuterium
fractionation. The very dense and cold ($n_{H_{2}}$ $\geq$10$^{6}$ cm$^{-3}$, T
$\leq$10 K) interior of the proto-BDs provide the suitable conditions for
efficient deuterium fractionation in these cores. There is no correlation
between the D/H ratios and the CO depletion factor, with the exception of the
DCN/HCN ratios that show a strong anti-correlation possibly due to the
difference in the peak emitting regions of the DCN and HCN molecules. Over a
wide range in the bolometric luminosities spanning $\sim$0.002--40 L$_{\odot}$,
we find a trend of higher DCO$^{+}$/HCO$^{+}$ (r = -0.7) and DCN/HCN (r = -0.6)
ratios, nearly constant DNC/HNC (r = -0.4) and DNC/HN$^{13}$C (r = -0.1)
ratios, lower N$_{2}$D$^{+}$/N$_{2}$H$^{+}$ ratios (r = 0.6) in the proto-BDs
compared to protostars. Only one Class II brown dwarf shows emission in the
DCO$^{+}$ (3-2) line. No correlation is seen between the D/H ratios and the
evolutionary stage.
