Superstring theories contain a large number of massless singlet fields. In this paper we examine their phenomenological and cosmological implications. In particular from the general constraints on their couplings we argue that many of them couple very weakly to matter fields. We further examine their impact on the big bang nucleosynthesis and derive a constraint on the number of gauge singlet fields that can be tolerated. We also consider the effects of decaying singlets on the overall mass density of the universe, entropy production and light element abundances.