Delphinium species are well-known toxic plants with diterpenoid alkaloid contents. There has been no previous investigation on the cytotoxicity of Delphinium formosum. The extracts of the different parts of D. formosum, an endemic species in Turkey, were investigated for their cytotoxic activity against the human liver carcinoma cell line (HepG2) and primary human umbilical vein endothelial cells (HUVEC). The cytotoxic effects of twelve extracts and subtractions were determined against HepG2 cells using the MTT assay. The only active extract was applied to the HUVEC as a model for healthy cells. Only one of the alkaloid-containing extracts from the aerial parts was toxic (IC50=244,9 mu g/mL against HepG2 and 144,4 mu g/mL against HUVEC), while the root extracts were inactive. The results were improbable although it is predicted secondary metabolites, such as diterpene alkaloids (methyllycaconitine, browniine, lycoctonine, avardharidine, antranoyllycoctonine, delsemine A/B and lycoctonine). Based on previous studies in the literature, the cytotoxic plants were also expected to exhibit insecticidal activity. Therefore, the cytotoxic extract of D. formosum was examined for its adulticidal and larvicidal activity against the yellow fever, dengue fever and the Zika virus vector Aedes aegypti L.