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 (between nulls) and an equivalent width of $7.1 pm 0.08$ km/s. This would imply a total HI column density of $2.6 pm 0.3 times 10^{21}$ per 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.
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