Ecdysteroid levels were determined by radioimmunoassay during the larval development of the spider crab Hyas araneus L. In each of the three larval instars (zoea I, zoea II, megalopa) a minimum in ecdysteroid levels was found during early postmoult and a maximum during premoult. The amounts of ecdysteroids increased during development and reached maxima of 93, 137, and 165 pg ecdysone equivalents per larva in zoea I, zoea II, and megalopa, respectively. Ecdysteroid concentrations per unit of body dry weight or carbon increased during each moulting cycle, but the average values showed decreasing tendency from the first to the last instar. The main ecdysteroid in all larval instars and during the moulting cycle was 20-OH-ecdysone, and practically no ecdysone could be detected by HPLC-RIA.