Abstract The inhibitory effect of folic acid antagonists was studied in Aedes aegypti larvae and adults. Aminopterin inhibited the growth of larvae, reared under sterile conditions on a chemically defined diet. Folinic acid was 135 times more effective than folic acid in overcoming the toxic effect of aminopterin. Aminopterin and methotrexate, in concentrations up to 2%, had no effect on oviposition when fed to adult mosquitoes prior to blood feeding. Egg production was inhibited only when 2% of methotrexate was fed in the protein meal; the inhibitory effect was more drastic when plasma, rather than whole blood, was fed. Methotrexate inhibition of oöcyte growth was most obvious in the early stages of oöcyte growth; the follicles which were not affected in the early stages were able to develop mature eggs, which hatched normally. There was no effect on oviposition in the gonotrophic cycles following methotrexate treatment. The relative resistance to methotrexate of adult mosquitoes, in comparison with other insects, is discussed.