  We present the catalog of HI sources extracted from the ongoing Arecibo
Legacy Fast ALFA (ALFALFA) extragalactic HI line survey, found within the sky
region bounded by 9h36m < RA < 11h36m and +08deg < DEC < +12deg. The HI catalog
presented here for this 118-deg^2 region is combined with ones derived from
surrounding regions also covered by the ALFALFA survey to examine the large
scale structure in the complex Leo region. Because of the combination of wide
sky coverage and superior sensitivity, spatial and spectral resolution, the
ALFALFA HI catalog of the Leo region improves significantly on the numbers of
low HI mass sources as compared with those found in previous HI surveys. The HI
mass function of the Leo I group presented here is dominated by low-mass
objects: 45 of the 65 Leo I members have M_HI < 10^8 Msun, yielding tight
constraints on the low-mass slope of the Leo I HI mass function. The best-fit
slope is alpha < -1.41 + 0.2 - 0.1. A direct comparison between the ALFALFA HI
line detections and an optical search of the Leo I region proves the advantage
of the ALFALFA strategy in finding low mass, gas-rich dwarfs. These results
suggest the existence of a significant population of low surface brightness,
gas-rich, yet still very low HI mass galaxies, and may reflect the same type of
morphological segregation as is seen in the Local Group. While the low mass end
slope of the Leo I HI mass function is steeper than that determined for
luminosity functions of the group, the slope still falls short of the values
predicted by simulations of structure formation in the LCDM paradigm.
