Botanical insecticides offer novel chemistries and actions that may provide effective mosquito control. Toosendanin (TSN, 95% purity) is one such insecticide used to control crop pests in China, and in this study, it was evaluated for lethal and sublethal effects on larvae and females of the yellowfever mosquito, Aedes aegypti (L.). TSN was very toxic to first instar larvae after a 24 h exposure (LC₅₀ = 60.8 µg/ml) and to adult females up to 96 h after topical treatment (LD₅₀ = 4.3 µg/female) or ingestion in a sugar bait (LC₅₀ = 1.02 µg/µl). Treatment of first instars for 24 h with a range of sublethal doses (6.3–25 µg/ml) delayed development to pupae by 1 to 2 d. Egg production and larval hatching from eggs were dose dependently reduced (>45%) by TSN doses (1.25–10.0 µg) topically applied to females 24 h before or 1 h after a bloodmeal. Ingestion of TSN (0.031–0.25 µg/µl of sugar bait) by females 24 h before a bloodmeal also greatly reduced egg production and larval hatch; no eggs were oviposited by females ingesting the highest dose. Further studies revealed that topical or ingested TSN dose-dependently disrupted yolk deposition in oocytes, blood ingestion and digestion, and ovary ecdysteroid production in blood-fed females. Overall, our results indicate that TSN is an effective insecticide for Ae. aegypti larvae and adults, because of its overt toxicity at high doses and disruption of development and reproduction at sublethal doses.