α-Mangostin derived from mangosteen was identified as a mosquito sterol carrier protein-2 inhibitor via high throughput insecticide screening. α-Mangostin was tested for its larvicidal activity against third instar larvae of six mosquito species, and the median lethal concentration values range from 0.84 to 2.90 ppm. The residual larvicidal activity of α-mangostin was examined under semifield conditions. The results indicated that α-mangostin was photolytic with a half-life of 53 min in water under full sunlight exposure. The effect of α-mangostin on activities of major detoxification enzymes such as P450, glutathione S-transferase, and esterase was investigated. The results showed that α-mangostin significantly elevated activities of P450 and glutathione S-transferase in larvae, whereas it suppressed esterase activity. Toxicity of α-mangostin against young rats was studied, and there was no detectable adverse effect at dosages as high as 80 mg/kg. This is the first multifaceted study of the biological activity of α-mangostin in mosquitoes. The results suggest that α-mangostin may be a lead compound for the development of a new organically based mosquito larvicide.