Although the crucial role of deep rooting on the hydraulic functioning of tropical trees has been pointed out, studies dealing with root development below 2 m are still scarce. Our study aimed to gain insight into the fine root traits of rainfed and irrigated orange trees down to the root front in deep tropical soils. Irrigation was applied during dry periods, only 3–15% more than the annual amounts of water supplied by rain. Fine roots were sampled down to a depth of 8 m on four dates in a randomized block design. The effects of soil depth and irrigation on major fine root traits, total fine root length and the relationship between leaf area index (LAI) and root area index (RAI) were studied. The total fine root mass was 728 g m−2 in rainfed plots and 536 g m−2 in irrigated plots during the driest period (Sept/Oct 2012). Across the four sampling dates, the mean depth of the root front was 6.1 m in rainfed plots and 5.5 m in irrigated plots close to the trees, and approximately 4.5 m in the inter-row covered by Brachiaria decumbens plants. LAI was little influenced by irrigation (about 4.5 m2 m−2), but mean RAI was 19.1 m2 m−2 in rainfed plots and 13.7 m2 m−2 in irrigated plots. Small irrigation rates reduced root development in very deep soil layers, and increased fruit production by 9% during the dry year. Deep rooting provides access to water stored in deep soil layers during the rainy season, and thus might have an important functional role during dry periods in tropical orange orchards. (Résumé d'auteur)