Temporal changes in the cuticular hydrocarbons of female Anopheles stephensi (Liston) (Diptera: Culicidae) were quantified using gas-liquid chromatography with flame-ionization detection. The ratio of two prominent hydrocarbons, nonacosane (C₂₉) and hentriacontane (C₃₁), was found to change significantly with respect to mosquito age over a period of 15 d. A regression model was developed using this ratio, C₂₉/C₃₁ = 3.96 − 1.63 log (age), and prediction intervals, based on a 12-d developmental interval necessary for females to transmit malaria, were generated using confidence levels for one-sided tests. The model predicted that females that had a C₂₉/C₃₁ ratio of 2.6 or greater were only 10% probable to be old enough to transmit malaria, whereas females with ratios of 1.8 or less were 90% probable.