A first-order phase transition is likely to have occurred in the early universe at the end of the quark era and the beginning of the hadron era. This phase transition would have created large amplitude isothermal fluctuations in the distribution of baryon number in the universe. These fluctuations have significant impact on the subsequent synthesis of isotopes of hydrogen, helium and lithium out of protons and neutrons. It turns out to be possible that all of the mass in the universe today is baryonic. © 1989.