  We have mapped the emission from atomic hydrogen at 21 cm from the galaxy NGC
3783 with the Australia Telescope Compact Array. Our main results are: a) the
HI morphology is irregular and perturbed, gathered in three blobs apparently
unrelated to the optical morphology; b) the observed HI velocity distribution
indicates a normal disk in differential rotation with a constant velocity out
to a radius of 160'' (30 kpc), c) the inclination of the disk is about 25 deg
with the kinematic major axis at a position angle slightly different from that
of the stellar bar, d) the HI mass inside a radius of 18'' is only 2.1 10^7
Msun, the total HI mass within 180'' is 1.1 10^9 Msun and the dynamical mass is
2 10^{11} Msun. The bulk of the gas in NGC 3783 is outside the diameter of the
stellar bar; e) Numerical simulations of the gas flow in the barred potential
derived from the red image indicate that the pattern speed is Omega_p = 38
km/s/kpc: the ring of Halpha emitting regions encircling the bar would then
correspond to UHR, and the Halpha accumulation in the center to a nuclear ring.
Various possibilities are discussed to account for the active nucleus fuelling.
