Resumo: Três experimentos foram conduzidos, um com infestação artificial (I) e outros dois com infestação natural (II, III). No artificial (Exp. I), os animais foram distribuídos em 9 grupos, com três repetições/tratamento. Cada animal foi infestado com 50 larvas em quatro incisões cutâneas, totalizando 200/animal. As larvas L1 foram infestadas com um intervalo de três a seis horas, de 24 e de 48 horas. Os grupos receberam os seguintes tratamentos: spinosad (50 e 150 ppm) e um controle sem tratamento. Os animais foram examinados com: 15, 30, 60 minutos e 6, 24 e 48 horas. Larvas que abandonavam as lesões foram colhidas e colocadas para pupar em BOD a 25°C e 90% de UR. Os resultados demonstraram que spinosad (50 e 125 ppm) atingiu 100% de eficácia no tratamento contra os três estádios larvários de C. hominivorax. A concentração de 125 ppm alcançou este valor mais rapidamente. Nos estudos com infestações naturais (Exp. II e III), os animais após receberem incisões cutâneas foram liberados a pasto para que ocorressem infestações. Detectada a presença de larvas, o animal era alocado, por sorteio, para um dos grupos experimentais. No experimento II utilizou-se spinosad 50 e 150 ppm e no experimento III 250 e 400 ppm. Como tratamentos positivos, foram utilizados: chlorpyriphos e uma associação de chlorphenvinphos + dichlorvos. Os animais foram examinados diariamente do dia zero (D0) até dia 12 (D12). Ficou demonstrado nos experimentos com infestação natural que spinosad 50 e 150 ppm apresentaram eficácia inferior a 100%, enquanto que a 250 e 400 ppm proporcionaram 100% de eficácia decorridas 24 horas do tratamento, a concentração com 400 ppm alcançou este valor decorridos 12 horas pós tratamento. / Abstract: Three experiments were conducted, the first one with an artificial infestation and the other two with natural infestation. In the first experiment (artificially infested) the animals were divided into 9 experimental groups, three repetition per treatment. Each animal was inoculated with 200 larvae in four skin incisions, two on the palette and two in the rump, each one containing 50 larvae. The placement of L1 larvae followed a range of 3 to 6 hours, 24 hours and 48 hours. The experimental groups had the following repetitions: spinosad 50 and 125 ppm, and a control group (untreated). After the treatment the animals were examined on the following period: 15, 30, 60 minutes, 6, 24 and 48 hours. Larvae which left the incision were taken for pupation on 25°C and 90% relativity humidity. The results demonstrated spinosad 50 and 125 ppm reached 100% effectiveness on the treatment of artificially caused myiasis by the three larval stages (L1, L2 and L3) of Cochliomyia hominivorax, in which the spinosad 125 ppm achieved it quicker. In the other experiments with natural infestation, after the skin incisions the animals were release on the field for the concurrency of the natural infestation. Once detected the presence of the larvae, the animals were randomly allocated in one of the experimental groups. On the experiment I were used the following concentration: spinosad 50 and 150 ppm on the experiment II spinosad 250 and 400 ppm. On both experiments were used two positives treatments: chlorpyriphos and a association of chlorphenvinphos plus dichlorvos. The animals were daily examined, from day zero (D0) to day 12 (D12).The results of the two experiments with natural infestation indicate that spinosad 50 and 150 ppm were not effective, showing effectiveness lower than 100%. However, spinosad 250 and 400 ppm reached 100% of efficacy were after 24 hours... (Complete abstract click electronic access below) / Orientador: Alvimar José da Costa / Coorientador: Gilson Pereira Oliveira / Banca: Luciano Melo de Souza / Banca: Giane Serafim da Silva / Mestre