Onychophorans possess a combination of annelidan and arthropodan characters and are always prominent in discussions of arthropod evolution. The chitinous cuticle is moulted periodically in juveniles as well as in adults. Using radioimrnunoassay (RIA) and high performance liquid chromatography (HPLC), the presence of fiee and conjugated ecdysteroids is reported in the body wall, digestive tract, slime gland and ovary/uterus of adult females ofEuperipatoides leuckartii.Within fiee ecdysteroids, ecdysone is dominant. The hctional significance of ecdysteroids is discussed in the light of the evolutionary history of moulting hormones.