The insecticidal activities of extracts from different parts of Cacalia tangutica (Franch.) H.-M. and two active ingredients, friedelin and stigmasterol, against Musca domestica L., Aedes albopictus (Skuse), and Spodoptera litura (F.) cells (SL cells) are reported. Extracts from leaves, flowers, and roots are highly active against adult M. domestica and the fourth-instar larvae of Ae. albopictus. The foliar extract was the most active with LC₅₀ values of 1.74 mg/g and 196.94 µg/ml against adult M. domestica and fourth-instar larvae of Ae. albopictus, respectively. Extracts from stems were the least active. Friedelin and rotenone were highly active and equitoxic to these two insect species. The LC₅₀ value of friedelin in adult M. domestica 48 h after treatment was 129.27 µg/g. The activity of stigmasterol on these two insect species was not obvious. However, the cytotoxicity of stigmasterol to S. litura cells was significantly greater than that of either friedelin or rotenone. At 25 µg/ml, the mortality of cells 24 h after treatment with stigmasterol was 23.0%. In contrast, friedelin and rotenone at the same concentration produced 16 and 18% mortality, respectively. Our results indicate differences between cytotoxicity in vitro and insect toxicity in vivo.