Cassava (Manihot esculenta Crantz) is the second most import crop in Mozambique after maize. It is a staple crop for more than 50% of the population in the northern provinces. It contributes about 30% of the carbohydrates required for human consumption. Cassava brown streak disease (CBSD) is the most economically important of the virus diseases that threaten cassava production within the smallholder sector in the country. The occurrence of CBSD in the north was reported for the first time in 1998 in Zambézia and later in Nampula and Cabo Delgado Provinces. CBSD affects both upper and underground parts of an infected plant; it causes root pulp necrosis, which makes roots unsuitable for consumption and marketing. High incidence of symptoms affecting up to 90% of the leaves and root-yield losses averaging 25% have been reported in the lowland coastal zones. Cassava production comprises a wide range of germplasm, which responds differently to CBSD infection. However, both local and improved varieties were observed with CBSD symptoms. A source of resistance has not been found yet. The Cassava Improvement Program in Mozambique started its activities in 2002 to address this problem. During surveys and visits in the affected areas, local landraces were collected that farmers perceived as being tolerant. Further evaluation and selection were conducted during two consecutive seasons. As a result, 17 local landraces and 1 improved clone (TMS 30001) were selected, which comprised the source population. The population was classified into three main groups according to traits: tolerance to CBSD, resistance to cassava mosaic disease (CMD), and good root cooking quality. Two designs, Polycross and Testar, were used in the crossing blocks. These arrangements were designed to make sure that all cultivars had a chance to cross. Two crossing methods were used in the field, open pollination and manual crossing. The crosses produced 4499 half-sib seeds and 47,671 full-sib seeds. During the crop season 2003/04, the seeds were planted in the seedling evaluation trial.