  We determined the HI mass function of galaxies in the Ursa Major association
of galaxies using a blind VLA-D array survey, consisting of 54 pointings in a
cross pattern, covering the centre as well as the outskirts of the Ursa Major
volume. The calculated HI mass function has best-fitting Schechter parameters
{\theta}^* = 0.19+/-0.11 Mpc^{-3}, log(M^*_{HI}/M_{\odot}) = 9.8+/-0.8 and
{\alpha} = -0.92+/-0.16. The high-mass end is determined by a complementary,
targeted WSRT survey, the low-mass end is determined by the blind VLA survey.
The slope is significantly shallower than the slopes of the HIPASS ({\alpha} =
-1.37+/-0.03+/-0.05) and ALFALFA ({\alpha} = -1.33+/-0.02) HI mass functions,
which are measured over much larger volumes and cover a wider range of cosmic
environments: There is a relative lack of low HI mass galaxies in the Ursa
Major region. This difference in the slope strongly hints at an environmental
dependence of the HI mass function slope.
