SUMMARYRoot 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. Tbe objective of tbis researcb was to examine colonization of Alliutn cepa L. roots at a range of P and N concentrations in order to clarify tbe effects of tbese nutrients on root colonization. In tbe first experiment, conducted under N-limiting conditions, P addition bad no effect on root colonization by Glomus etunicatum Becker & Gerdemann and increased root colonization by Gigaspora margarita Gerdemann and Trappe. In tbe second experiment, conducted under N-sufficient conditions, P addition suppressed colonization by G. etunicatum. In tbe tbird experiment, P and N additions were sucb tbat tissue P:N ratios ranged from 0-03 (P limiting) to 0-17 (N limiting). No relationsbip was found between P:N ratio and root colonization for two isolates of G. etunicatum. Root colonization was not affected by P addition wben plants were deficient in N but, wben N was sufficient, P addition suppressed root colonization. Tbese results demonstrate tbat plant N stress affects tbe resistance of tbe bost root to colonization by VA mycorrbizal fungi.