Vesicular-arbuscular mycorrhizal (M) fungal colonization, growth, and nonstructural carbohydrate status of sour orange (Citrus aurantium L.) seedlings were compared at low- and high-phosphorus (P) supply following inoculation with four Glomus isolates: G. intraradices (Gi, FL208), G. etunicatum (Ge, UT316), G. claroideum (Gc, SC186), and Glomus sp. (G329, FL906). Nonmycorrhizal (NM) seedlings served as controls. At low-P supply, increases in incidence of M colonization, vesicles and accumulation of fungal fatty acid 16:1omega(5)C in roots were most rapid for G329-inoculated seedlings, followed closely by Gi- and Gc-inoculated seedlings. Glomus etunicatum was a less aggressive colonizer and produced lower rates of fungal fatty acid accumulation in seedling roots than the other Glomus species. Nonmycorrhizal and Ge-inoculated seedlings had lower P status and growth rates than seedlings inoculated with Gi or G329. Glomus claroideum increased seedling P status, but growth rate was lower than for seedlings colonized by Gi or G329, suggesting higher belowground costs for Gc colonization. In P-sufficient roots colonized by Gi, Gc, or G329, starch and ketone sugar concentrations were lower than in P-deficient NM and Ge-inoculated plants. Under conditions of high-P supply where mycorrhizae provided no P benefit to the seedlings, colonization by Gc, Gi, and G329 was delayed and reduced compared to that at low-P supply; however, the relative colonization rates among Glomus spp. were similar. Colonization by Ge was not detected in roots until 64 days after inoculation. Compared to NM seedlings, growth rates of mycorrhizal seedlings were reduced by the three aggressive fungi but not by the less aggressive Ge. After 64 days, starch and ketone sugar concentrations were lower in fibrous roots colonized by Gc, Gi, and G329 than in NM roots, indicating greater utilization of nonstructural carbohydrates in roots colonized by the aggressive fungi. After 49 days, colonization by the aggressive fungi increased root biomass allocation which may have contributed to the lower growth rate of mycorrhizal seedlings compared to NM seedlings. Thus, Glomus spp. that were aggressive colonizers of roots at low-P supply were also aggressive colonizers at high-P supply, resulting in greater belowground C costs and growth depression compared with the less aggressive colonizer, Ge.