Arbuscular mycorrhizal relationsliips link plant root systems and zygomycetous fungi from the family Glomales in a symbiotic association which provides a carbon source to the fungus and benefits to the plant in terms of nutrient and water acquisition, and disease resistance. Root system colonisation is characterised by the formation of intracellular hyphae and internal fungal structures, the arbuscles and vesicles. Colonisation occurs after root penetration by fimgal hyphae originating from either spores or extra-radical hyphae. Tire extra-radical hyphae, which form the fungal mycelium in the soil and have a primary role in host plant nutrition, are a largely neglected feature of the symbiosis due to the difficulties involved in their study. In the present work, methodologies were developed to allow in vitro studies of extra-radical hyphae and thus circumvent problems associated with the inaccessibility and opacity of the soil environment. Root pieces colonised by the arbuscular mycorrhizal (AM) fungus Glomus etunicatum Becker & Gerdemann (S329, INVAM, USA) were inoculated onto dialysis membrane overlying transparent high purity agarose gel. This restricted the hyphae to a two-dimensional growth form which facilitated observation using microscopy and image analysis techniques, and enabled morphological measurements to be taken without the added complexity of three dimensional growth. The two major influences 011 AM fimgal growth are those of soil and of plant origin. The present work studied the effects of plant related factors on AM fungal hyphae. Host and non-host plant factors, and derivatives of these, were found to influence hyphal growth as characterised both by length measurements and morphological parameters. Host root exudates reduced hyphal growth, apparently as a result of changes in the overall distribution of hyphae within the mycelium, as measured by fractal dimension (FD). Non-host exudates and plant flavonoid compounds also decreased hyphal growth, but this result could not be attributed to changes in either branching or mycelial organisation. Root extracts of host and non­host plants did not significantly affect hyphal length, but did affect mycelial morphology. Although host root extracts appeared to have no effect 011 branching, they did alter hyphal distribution within the mycelium, as evidenced by an increase in FD. Non-host root extracts increased both branching and FD. Colonised host plant root extracts increased both hyp ha I length and branching, but had 110 significant effect on hyphal distribution within the mycelium. Results are discussed in the context of previous work earned out on germ tube hyphae of AM fungi from spore inoculum. Observations of extra-radical hyphae showed differential responses when compared with germ tube hyphae more commonly studied by previous authors. This indicates that different phases of the fungal life cycle respond in different ways to similar environmental influences.