  We report the non-detection of the [CII] 157.74 um transition in the z=6.42
quasar SDSS J1148+5251 after 37.5 hours of integration with the James Clerk
Maxwell Telescope. This transition is the main cooling line of the star-forming
interstellar medium, and usually the brightest FIR line in galaxies. Our
observed RMS of 1.3 mK in the Ta* scale translates to L([CII])<2.6 x 10^9 Lsun.
Using a recent estimate of the far-infrared continuum of this quasar, we derive
for SDSS J1148+5251 L([CII])/L(FIR)<5 x 10^-4, a ratio similar to that observed
in local ultra-luminous infrared galaxies but considerably smaller than what is
typical in nearby normal and starburst galaxies. This indicates that the small
L([CII])/L(FIR) ratio observed locally in luminous far-infrared objects also
persists at the highest redshifts.
