We have used the Westerbork Synthesis Radio Telescope to detect HI 21cm absorption at z \sim 0.7645 in the gravitational lens system towards PMN J0134 - 0931 .
The 21cm profile has two broad components , with peak optical depths of 0.047 \pm 0.007 and 0.039 \pm 0.007 , at heliocentric redshifts 0.76470 \pm 0.00006 and 0.76348 \pm 0.00006 , respectively .
The redshift of the stronger component matches that of CaII H and K absorption detected earlier .
The absorption has a total velocity width of \sim 500 km s ^ { -1 } ( between nulls ) and an equivalent width of 7.1 \pm 0.08 km s ^ { -1 } .
This would imply a total HI column density of 2.6 \pm 0.3 \times 10 ^ { 21 } cm ^ { -2 } , for a spin temperature of 200 K and a covering factor of unity .
The high estimated HI column density is consistent with the presence of large amounts of dust at the lens redshift ; the intervening dust could be responsible for the extremely red colour of the background quasar .