. Affandi . 2008 . Collection and Identification of Predatory Mites (Ascidae: Asca) and its Population on Mandarin Citrus Ecosystem. The objectives of the research were to find out the type of genera Asca predatory mites and its population on ecosystem of mandarin citrus. A purposive sampling survey method was conducted at a mandarin citrus orchard at Aripan Research Station of the Indonesian Tropical Fruits Research Institute, Solok, West Sumatera in the periode of September 2003 to July 2004. The results showed that there were 18 species of genera Asca predatory mites with total number of 3,919 were collected and identified. Among them, predatory mites Asca longiseta, A. labrusca, A. vulgaris, A. butuanensis, and A. breviseta were the most populous from the highest to the lowest, respectively. The most preferable habitat of genera Asca predatory mites was the weed under the canopy of citrus (average 2.33/sample), followed by the canopy of citrus (average 0.75/sample), and the plant wates under the canopy of citrus (average 0.51/sample). Among the weed, Chromolaena odorata was the most preferable habitat of genera Asca predatory mites with average population of 5 predatory mites per sample (75 g). Predatory mites tend to migrate to weed habitat when the population of prey in the canopy of citrus was low. The results of this research was useful to determine the type of predatory mites especially on genera Asca that has potential as biological control against phytophagous mites.