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Fertilized oocytes of the brine shrimpArtemia franciscanaundergo either ovoviviparous or oviparous development, yielding free-swimming larvae (nauplii) or encysted gastrulae (cysts), respectively. Encystment is followed by diapause, wherein metabolism is greatly reduced; the resulting cysts are very resistant to extreme stress, including desiccation and long-term anoxia. The synthesis of p26, a small heat shock/α-crystallin protein produced only in oviparously developingArtemia,is shown in this paper to be transcriptionally regulated. A p26 mRNA of about 0.7 kb was detected on Northern blots in the second day after oocyte fertilization. It peaked as embryos encysted and declined rapidly when activated cysts resumed development. The appearance of p26 protein, as indicated by immunoprobing of Western blots, followed mRNA by 1 day; it also increased as encystment occurred but remained constant during …