Different levels of Aphis gossypii Glover were maintained on clementine trees by means of isolation inside mesh cages. Clementine yield loss was correlated with the number of aphids per square meter of canopy, obtaining the corresponding formula. Starting from this formula, the economic injury level (EIL) was calculated as follows: [218,333C 11,130.6 · VP₀K]/[57.08 · VP₀K − 100C] = EIL (aphids/m²), where C = total insecticide cost per hectare, V = fruit price per kilogram, P₀ = yield per hectare of a minimum pest level orchard, and K = reduction of injury due to treatment. Starting from the EIL, the economic threshold, the environmental EIL, and the environmental economic threshold were obtained. Formulae relating the number of aphids per square meter with other simpler indices expressing these thresholds (percentage of infested shoots, infested shoots per 0.25 m² ring, index of infestation, percentage of occupied rings, and number of weekly captured aphids in a yellow trap) also were calculated.