The effects of cyromazine on egg production and subsequent egg-to-adult survival were examined on susceptible, heterozygous, and homozygous cyromazine-resistant genotypes of the Australian sheep blowfly Lucilia cuprina (Wiedemann) by administering to adults 10 ppm of cyromazine in drinking water. Cyromazine reduced egg production, hatch, and subsequent larval survival in susceptible genotypes by acting at the embryonic stage of development. Resistance negated these effects dominantly for egg production and egg hatch and in a partially dominant manner for egg-to-adult survivorship.