1. The change in the properties of high-strength cast iron in relation to isothermal holding time is extremal in character. 2. The best mechanical properties of cast iron with spheroidal graphite are obtained at 300–400° with 25–40% retained austenite. 3. Cast iron with spheroidal graphite whose matrix consists of a structural mixture of bainite, martensite, and retained austenite, with a predominance of martensite, is susceptible to premature failure. 4. The reduction of the amount of retained austenite with prolonged isothermal holding times leads to a reduction of the ductility; the strength characteristics decrease negligibly in this case. 5. The isothermal bainitic transformation is divided into three stages that differ in the average rate of the transformation.