1. A broad group of acid hydrolytic activities have been determined during Artemia development including cathepsin B acid ribonuclease, acid deoxyribonuclease, acid phosphatase, acid phosphodiesterase, β-glucosidase, β-N-acetylgalactosaminidase and acid lipase. These enzymes present maximum activity in nauplii, when yolk degradation is maximum. 2. Artemia cathepsin B proteinase is able to degrade lipovitellin in vitro with a pattern similar to that found in vivo. These results suggest the involvement of acid hydrolases in the degradation of yolk. 3. In cryptobiotic embryos, acid hydrolases were found associated with structures of buoyant density 1.18. The structures appeared at the electron microscope as 0.4 μm dia. vesicles when revealed by acid phosphatase cytochemistry. This work has been supported by a grant from Comisión Asesora de lnvestigación Científica y Técnica. R.P. had a fellowship from Caja de Ahorros de Madrid.