We estimate the two-point correlation function in redshift space of the recently compiled HIPASS neutral hydrogen ( HI ) sources catalogue , which if modeled as a power law , \xi ( r ) = ( r _ { 0 } / r ) ^ { \gamma } , the best-fitting parameters for the HI selected galaxies are found to be r _ { 0 } = 3.3 \pm 0.3 h ^ { -1 } Mpc with \gamma = 1.38 \pm 0.24 .
Fixing the slope to its universal value \gamma = 1.8 , we obtain r _ { 0 } = 3.2 \pm 0.2 h ^ { -1 } Mpc .

Comparing the measured two point correlation function with the predictions of the concordance cosmological model ( \Omega _ { \Lambda } = 0.74 ) , we find that at the present epoch the HI selected galaxies are anti-biased with respect to the underlying matter fluctuation field with their bias value being b _ { 0 } \simeq 0.68 .
Furthermore , dividing the HI galaxies into two richness subsamples we find that the low mass HI galaxies have a very low present bias factor ( b _ { 0 } \simeq 0.48 ) , while the high mass HI galaxies trace the underlying matter distribution as the optical galaxies ( b _ { 0 } \simeq 1 ) .
Using our derived present-day HI galaxy bias we estimate their redshift space distortion parameter , and correct accordingly the correlation function for peculiar motions .
The resulting real-space correlation length is r ^ { re } _ { 0 } = 1.8 \pm 0.2 h ^ { -1 } Mpc and r ^ { re } _ { 0 } = 3.9 \pm 0.6 h ^ { -1 } Mpc for the low and high mass HI galaxies , respectively .
The low-mass HI galaxies appear to have the lowest correlation length among all extragalactic populations studied to-date .
In order to corroborate these results we have correlated the IRAS-PSCz reconstructed density field , smoothed over scales of 5 h ^ { -1 } Mpc , with the positions of the HI galaxies , to find that indeed the HI galaxies are typically found in negative overdensity regions ( \delta \rho / \rho _ { PSCz } \raise - 3.0 pt \hbox { \hbox to 0.0 pt { \hbox { $ \sim$ } } % \raise 4.0 pt \hbox { $ < $ } } 0 ) , even more so the low HI-mass galaxies .

Finally , we also study the redshift evolution of the HI galaxy linear bias factor and find that the HI-galaxy population is anti-biased up to z \sim 1.3 .
While at large redshifts z \sim 3 , we predict that the HI galaxies are strongly biased .
Our bias evolution predictions are consistent with the observational bias results of Lyman- \alpha galaxies .

Keywords : galaxies : clustering - HI sources - cosmology : theory - large-scale structure of universe