Mycosporines, N-substituted amino acid derivatives of 2-hydroxymethyl-2-methoxycyclohex-1-en-3-one, were isolated from dormant embryos (cysts) of Artemia. The major component in all strains was mycosporine (-Gly:-Ser). Small amounts of mycosporines containing glycine and threonine, glycine and alaninol as well as glycine and aspartic acid were identified in cysts of a strain from Brazil. Free amino acids were released from mycosporines by treatment with bromine water or an iodine peroxidase (EC 1.11.1.8). The amount of mycosporines in developing cysts decreased while nauplii contained only gadusol (1,4,5-trihydroxy-5-hydroxymethyl-2-methoxycyclohex-1-en-3-one).