AbstractInsects convert ecdysone and 20‐hydroxyecdysone into their corresponding 26‐oic derivatives, named ecdysonoic acid and 20‐hydroxyecdysonoic acid respectively. The conversion takes piace in several tissues and can either be the only pathway for converting ecdysone into highly polar ecdysteroids, or coexist with various conjugating mechanisms.20‐Hydroxyecdysonoic acid was isolated fromPieris brassicaepupae as its methyl ester derivative. Its chemical structure was identified by Cl/D mass spectrometry and compared with a synthetic compound (20‐hydroxy‐25‐deoxyecdysonoic acid) chemically prepared by oxidation of inokosterone (20,26‐dihydroxy‐25‐deoxyecdysone). Natural ecdysonoic acids appear to exist as a mixture of 25R and 25S isomers.The significance of this pathway is discussed in comparison with similar reactions occuring in the metabolism of steroid horm