A strong longitude variation in the midlatitude H Lyman &agr; brightness of Jupiter has been discovered using data from the Voyager ultraviolet spectrometers. A minimum brightness of 14.4 kR was observed in the System III [1965> west longitude range 200 ¿--300 ¿. The intensity rises to a broad peak of 19.6 kR near a longitude of 100 ¿, a 36% increase over the minimum. This intensity ''bulge'' has observed to rotate with the planet and appeared at the same longitude during both Voyager encounters, four months apart. This is surprising discovery; no other planet studied to date has such a characteristic and the upper levels of the Jovian atmosphere were expected to be uniform in longitude. It is likely that a longitudinal asymmetry in charged particle precipitation leads to selective heating of the upper atmosphere and increased dissociation of H2, increasing the hydrogen column abundance. The resulting increase in resonant scattering could produce the observed intensity bulge.