Four cDNA clones coding for different Artemia actin isoforms have been isolated. Three of the clones contain the complete coding sequences while the fourth one lacks 145 bases, coding for the 49 amino terminal amino acids of the protein. The amino acid sequences predicted for the four actin isoforms identified are highly homologous to insect actins as well as to vertebrate cytoplasmic actins. The four identified cDNA clones code for mRNAs of 5.2, 1.9, 1.6 and 1.8 kb, respectively, whose expression is regulated during development. Three of the actin mRNAs are present in cryptobiotic embryos while the other is not. The steady-state levels of all four mRNAs increase during development to reach maximal levels by 10-15 hours of development and decrease thereafter. The total number of actin genes encoded in the Artemia genome has been estimated as 8 to 10 by Southern analysis of total DNA.