The composition of the jovian atmosphere from 0.5 to 21 bars along the descent trajectory was determined by a quadrupole mass spectrometer on the Galileo probe. The mixing ratio of He (helium) to H_2 (hydrogen), 0.156, is close to the solar ratio. The abundances of methane, water, argon, neon, and hydrogen sulfide were measured; krypton and xenon were detected. As measured in the jovian atmosphere, the amount of carbon is 2.9 times the solar abundance relative to H_2, the amount of sulfur is greater than the solar abundance, and the amount of oxygen is much less than the solar abundance. The neon abundance compared with that of hydrogen is about an order of magnitude less than the solar abundance. Isotopic ratios of carbon and the noble gases are consistent with solar values. The measured ratio of deuterium to hydrogen (D/H) of (5 ± 2) x 10-5 indicates that this ratio is greater in solar-system hydrogen than in local interstellar hydrogen, and the ^3He/^4He ratio of (1.1 ± 0.2) x 10-4 provides a new value for protosolar (solar nebula) helium isotopes. Together, the D/H and ^3He/^4He ratios are consistent with conversion in the sun of protosolar deuterium to present-day ^3He.