  We investigate Big Bang nucleosynthesis (hereafter, BBN) in a cosmic
environment characterized by a distribution of small-scale matter/antimatter
domains. Production of antimatter domains in a baryo-asymmetric universe is
predicted in some electroweak baryogenesis scenarios. We find that cosmic
antimatter domains of size exceeding the neutron-diffusion length at
temperature T approx. 1 MeV significantly affect the light-element production.
Annihilation of antimatter preferentially occurs on neutrons such that
antimatter domains may yield a reduction of the He-4 abundance relative to a
standard BBN scenario. In the limiting case, all neutrons will be removed
before the onset of light-element production, and a universe with net baryon
number but without production of light elements results. In general, antimatter
domains spoil agreement between BBN abundance yields and observationally
inferred primordial abundances limits which allows us to derive limits on their
presence in the early universe. However, if only small amounts of antimatter
are present, BBN with low deuterium and low He-4, as seemingly favored by
current observational data, is possible.
