  Recent models invoking extra spacelike dimensions inhabited by (bulk)
neutrinos are shown to have significant cosmological effects if the size of the
largest extra dimension is R > 1 fm. We consider effects on cosmic microwave
background anisotropies, big bang nucleosynthesis, deuterium and Li-6
photoproduction, diffuse photon backgrounds, and structure formation. The
resulting constraints can be stronger than either bulk graviton overproduction
constraints or laboratory constraints.
