Aedes aegypti may be considered the most dangerous animal to the human being due to the number and severity of the diseases it transmits. We evaluated the relative toxicity of spinosad against eggs, larvae, pupae, and females of the New Orleans strain of this species. Initially, we estimated the concentrations that caused between 0 and 100% mortality, and then intermediate concentrations that covered this range were included. Five replications were carried out and each replication included an untreated control. The percentage of egg hatching inhibition (eggs) and percentage mortality (larvae, pupa, and adult female) was recorded after 24 hours of exposure. The LC50 for eggs was 28.9 mg liter(-1), the data modeled a straight line (R-2 = 0.9747) and the highest percentage of egg hatching inhibition was 70%. The LC50 for each instar larvae was: 1 = 0.009 mg liter(-1), 2 = 0.017 mg liter(-1), 3 = 0.08 mg liter(-1), and 4 = 0.12 mg liter(-1). In the same order, the LC95 values were 0.064, 0.14, 0.28, and 0.62 mg liter(-1), respectively. Both at the LC50 and LC95 levels, there were not significant differences between the third and the fourth instars. The LC50 and LC95 against the pupal stage were 30.4 and 475.1 mg liter(-1). The LC50 and LC95 values for females were 0.005 and 0.04 mg cm(-2). It is concluded that spinosad is toxic to all biological stages of A. aegypti.