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The arid and semi-arid region occupies 89.3% of north-east Brazil, and livestock farming is one of the most important activities in this region. Experiments were conducted in four locations in the semi-arid of Pernambuco, Brazil. The objective was to study N (0, 75, 150, 225, and 300 kg N ha(-1) yr(-1)) and P fertilizations (0 and 33 kg P ha(-1) yr(-1)) at two populations (5000 and 40 000 plants ha-1) of Opuntia ficus-indica (L.) Mill. Increasing population from 5000 to 40 000plantsha-1 increased productivity in all four sites. Phosphorus fertilization increased dry matter yield (DMY) only when soil P was < 10 mg kg(-1). Nitrogen fertilization increased linearly DMY in the higher population, but no response to N was observed with 5000 plants ha(-1). Nitrogen fertilization also increased plant N concentration and rainfall use efficiency. Cladode area index (CAI) increased with population but even in the higher population the CAI was I or less. In summary, increasing plant population and fertilizing with N and P resulted in greater productivity of O. ficus-indica in north-east Brazil. Phosphorus fertilization involved less economical risk to the farmer but positive responses occurred only when soil P was < 10 mg kg(-1). (c) 2006 Elsevier Ltd. All rights reserved.