Diadenosine 5′,5‴P1, P4- tetraphosphate (Ap has been detected in cysts and developing embryos of the brine shrimpArtemiain amounts 104-106times lower than that of the guanine analogue Gp4G. The unexpectedly high level of Ap4A in dormant cysts of 2.37 pmol/106cells can be reduced to 0.03 pmol/106cells by decapsulatlon and storage in saturated NaCl. When development is reinitiated, the Ap4A content of the decapsulated embryos undergoes a rapid 125 -fold increase, reaching a maximum of 3.79 pmol/106cells at the point of emergence when DNA replication begins. If replication is delayed by hypoxia, the Ap4A level is adjusted in order to reach the same maximum value when replication finally begins. As replication proceeds, the level of Ap4A declines again. Unlike mammalian cells, Ap4A InArtemiais less metabolically labile than ATP. These results are consistent with the suggested role of Ap4A in the initiation of DNA synthe