Productivity of irrigated prickly pear cactus [Opuntia ficus-indica (L.) Miller] was studied over 3 years in central Chile using two planting densities. A low-density planting (0.25 plants/m2), traditionally favored for fruit production, had maximal fruit productivity in the 2nd year (6 Mg dry weight/ha per year). A high-density planting (24 plants/m2), which assured almost full interception of incident solar radiation, led to an extremely high shoot dry-weight productivity (50 Mg·ha-1·year-1) in the 2nd year and maximal fruit productivity (6 Mg·ha-1·year-1) in the 3rd year. Cladode dry weight tended to increase with cladode surface area. However, fruit production did not occur until the dry weight per cladode exceeded the minimum dry weight for a particular cladode surface area by at least 33 g. The year-to-year variation in fruit production apparently reflected variations in such excess dry weight and, hence, in the storage reserves of individual cladodes.