Root colonization by vesicular-arbuscular (VA) mycorrhizal fungi is often suppressed by phosphorus (P) additions; however, nitrogen (N) additions have been reported to both stimulate and suppress root colonization. The objective of this research was to examine colonization of Allium cepa L. roots at a range of P and N concentrations in order to clarify the effects of these nutrients on root colonization. In the first experiment, conducted under N-limiting conditions, P addition had no effect on root colonization by Glontus etunicatum Becker & Gerdemann and increased root colonization by Gigaspora margarita Gerdemann and Trappe. In the second experiment, conducted under N-sufficient conditions, P addition suppressed colonization by G. etunicatum. In the third experiment, P and N additions were such that tissue P:N ratios ranged from 0–03 (P limiting) to 017 (N limiting). No relationship was found between P:N ratio and root colonization for two isolates of G. etunicatum. Root colonization was not affected by P addition when plants were deficient in N but, when N was sufficient, P addition suppressed root colonization. These results demonstrate that plant N stress affects the resistance of the host root to colonization by VA mycorrhizal fungi.