Let k be a number field, f (x) ∈ k[x] a polynomial over k with f (0) = 0, and ᏻ * k,S the group of S-units of k, where S is an appropriate finite set of places of k. In this note, we prove that outside of some natural exceptional set T ⊂ ᏻ * k,S , the prime ideals of ᏻ k dividing f (u), u ∈ ᏻ * k,S \ T , mostly have degree one over ;ޑ that is, the corresponding residue fields have degree one over the prime field. We also formulate a conjectural analogue of this result for rational points on an elliptic curve over a number field, and deduce our conjecture from Vojta's conjecture. We prove this conjectural analogue in certain cases when the elliptic curve has complex multiplication.