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In this contribution, we present some preliminary observational results fromthe completed ultra-deep survey of 21cm emission from neutral hydrogen atredshifts z=0.164-0.224 with the Westerbork Synthesis Radio Telescope. In twoseparate fields, a total of 160 individual galaxies has been detected inneutral hydrogen, with HI masses varying from 1.1x10^9 to 4.0x10^10 Msun. Thelargest galaxies are spatially resolved by the synthesized beam of 23x37arcsec^2 while the velocity resolution of 19 km/s allowed the HI emission linesto be well resolved. The large scale structure in the surveyed volume is tracedwell in HI, apart from the highest density regions like the cores of galaxyclusters. All significant HI detections have obvious or plausible opticalcounterparts which are usually blue late-type galaxies that are UV-bright. Oneof the observed fields contains a massive Butcher-Oemler cluster but none ofthe associated blue galaxies has been detected in HI. The data suggest that thelower-luminosity galaxies at z=0.2 are more gas-rich than galaxies of similarluminosities at z=0, pending a careful analysis of the completeness near thedetection limit. Optical counterparts of the HI detected galaxies are mostlylocated in the 'blue cloud' of the galaxy population although several galaxieson the 'red sequence' are also detected in HI. These results hold great promisefor future deep 21cm surveys of neutral hydrogen with MeerKAT, APERTIF, ASKAP,and ultimately the Square Kilometre Array.

Westerbork Ultra-Deep Survey of HI at z=0.2