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The "ALFA Ultra Deep Survey" (AUDS) is an ongoing 21-cm spectral survey withthe Arecibo 305m telescope. AUDS will be the most sensitive blind surveyundertaken with Arecibo's 300 MHz Mock spectrometer. The survey searches for21-cm HI line emission at redshifts between 0 and 0.16. The main goals of thesurvey are to investigate the HI content and probe the evolution of HI gaswithin that redshift region. In this paper, we report on a set of precursorobservations with a total integration time of 53 hours. The survey detected atotal of eighteen 21-cm emission lines at redshifts between 0.07 and 0.15 in aregion centered around ra~0:00h, dec= 15:42deg. The rate of detection isconsistent with the one expected from the local HI mass function. The derivedrelative HI density at the median redshift of the survey isrho_HI[z=0.125]=(1.0+/-0.3)*rho_0, where rho_0 is the HI density at zeroredshift.

Deep 21-cm HI Observations at z~0.1: The Precursor to the Arecibo Ultra Deep Survey