Most of the world's sugar is produced from a semi-perennial plant whose root system, although being one key to its success, remains poorly understood. In this study, we sought to describe how nitrogen fertilisation is likely to affect the development of the sugarcane root system and can have significant agronomic implications. We studied sugarcane root distribution, root biomass production and root N accumulation from soil cores sampled down to a depth of 1 m throughout the growth cycle of a ratoon crop in a 144 kgN ha-1 year fertilised plot and at crop harvest in six ratoon plots with fertilisation ranging from 0 to 330 kgN ha-1 year. The development of the root system in the fertilised sugarcane plot showed 1/homogeneous colonization of the topsoil by fine roots from the early phase, followed by 2/a progressive development of thicker roots mainly localised under the sugarcane row. The results suggested that nitrogen fertilisation could reduce root density in the topsoil layer. The root-to-shoot ratio of biomass and N mass decreased, respectively, from 2.1 to 0.3 and from 1.2 to 0.7 throughout an annual crop growth cycle in a fertilised plot. When sugarcane was not fertilised, an increase of 70% root biomass was observed as compared to fertilised sugarcane. In addition, approximately half of the cane N mass was found in the root compartment of the non-fertilised crop. The root system of sugarcane appeared to be a major pool of N that should be considered in studies dealing with fertiliser N use efficiency and N cycling in sugarcane agroecosystems.