Actively collimated balloon‐borne scintillation counters employing a special phoswich anticoincidence technique were flown a total of 5 times from Palestine, Texas. Jupiter was observed for a total of 133 min, and an upper limit to the flux of X rays present at the observation time is 1.6 × 10−2X rays/cm² sec in the energy range 30‐100 kev. Three separate calculations are made to estimate the flux of Jovian X rays at the earth; the first two calculations involve scaling up terrestrial precipitating electron fluxes by factors that depend on the energy intercepted by Jupiter's magnetosphere and the Jovian magnetic‐field strength, whereas the third estimate is based on a model of Jovian decametric emission. These estimates range from 10−9to 10−1X rays/cm² sec in the energy range 30‐100 kev. It is concluded that, since there was no decametric emission at the time of the flight and there had been no significant solar activity for several days prior to the flight, no X rays were being generated at the time of