When Myrica cerifera was grown in a number of 1/1-strength Hoagland's solutions, each lacking one mineral nutrient, cluster roots formed only in solutions lacking phosphorus (P). In seedlings initially fertilized with various P concentrations, cluster root formation decreased with increasing P supply and was totally suppressed in solutions containing ≥ 1 mg P l−1. As total root weight did not vary significantly between P treatments, the proportion of the root system expressed as cluster roots or non-cluster roots varied with phosphorus supply. A similar response to increasing levels of foliar-applied P suggests that it may be internal P concentration in the plant that determines the initiation of cluster roots and not external P levels in the soil. Nitrogen, supplied as fixed dinitrogen or nitrate, did not have any effect on development of cluster roots and it appears that the capacity of actinorhizal plants to fix nitrogen by symbiotic association with a soil micro-organism is of no significance to cluster root formation. While abundant root hair development and extensive mycorrhizal mycelium distribution in the rhizosphere may represent more efficient morphological forms of improving plant P uptake, the significance of cluster roots in relation to P nutrition and the mechanisms involved in their development warrant further investigation.