1 The effects of three different isolates of co-occurring arbuscular mycorrhizal fungi (AMF) from the same calcareous grassland on clonal growth traits of Prunella vulgaris and P. grandiflora were investigated. 2 P. vulgaris and P. grandiflora were found to be highly dependent on AMF for growth in soil from the same calcareous grassland. The addition of phosphorus did not significantly increase the growth of either nonmycorrhizal or mycorrhizal P. vulgaris. 3 The three AMF isolates differed significantly in their effects on size of the two plant species. The AMF isolates also differed in their effects on stolen branching and stolen length and here the effects were not the same in the two plant species The differential effects of AMF isolates on clonal growth traits were consistently independent of the level of AMF colonization throughout the experiment indicating that the effects were not due to different rates of colonization. Some of the AMF isolate effects on stolen branching in P. vulgaris is were also independent of the effects of AMF isolates on plant size. These results suggest that different AMF in a natural community have the potential to influence the growth number of ramets and distribution of ramets in Prunella populations. 4 The strong differential effects of AMF isolates on clonal growth traits of P. vulgaris occurred in a homogeneous environment. Plasticity in such traits has previously been considered important for efficient resource foraging in a heterogeneous environment. Our results however indicate that different AMF in a community could strongly influence resource foraging strategies in clonal plants irrespective of environmental heterogeneity. The results also suggest that increasing the availability of phosphorus would be unlikely to alter these effects. 5 Our results point strongly to the potential importance of AMF diversity as a determinant of plant population structure in ecosystems.