AbstractAs an adjunct to biochemical studies on the action of juvenile hormone antagonists (‘antijuvenile hormones’), a study has been made of the effects on insect development of 71 compounds based on the 2,2‐dimethylchromene structure, and 30 other compounds, including especially chemicals containing acetylenic and 1,3‐benzodioxole (methylenedioxyphenyl) groups, that were expected to inhibit microsomal mixed function oxidases. The ageratochromenes, 7‐methoxy‐2,2‐dimethylchromene and 6,7‐dimethoxy‐2,2‐dimethylchromene (precocene I and precocene II, respectively), were used for comparison. With the jar deposit method used forOncopeltus fasciatus, and the topical application method used forLocusta migratoria, only derivatives of 2,2‐dimethylchromene and 2‐(3‐methylbut‐2‐enyl)phenol, the alkene double bonds of which were activated by an alkoxy group, exhibited characteristic antijuvenile hormone actions. In the whole life‐cycle exposure test onO. fasciatus, a number of other compounds had non‐specific effects on development, especially at the fifth instar. Some of the compounds, including certain derivatives of 2,2‐dimethylchromene with ester side‐chains, had juvenile hormone agonist actions. The scope for structural optimization for antijuvenile hormone action in the 2,2‐dimethylchromene series appears to be limited, and structure‐activity relationships are obscured, especially forO. fasciatus, by an overlap between the prothetelic morphogenetic and lethal effects. The results are discussed in relation to the m