Two cloned Eco RI fragments have been isolated from a Xenopus borealis genomic library in phage lambda, using an actin cDNA clone as a probe. Both fragments have been restriction-mapped and partially sequenced. As a result of this, one fragment, 2.6 kb in size, has been found to encode an amino acid sequence identical to the first 149 amino acids of the mammalian cardiac muscle actin, while the other, 3.6 kb in size, encodes a protein which is almost identical to the first 267 amino acids of the mammalian beta-cytoskeletal actin. The putative muscle actin gene contains introns at different positions to those of the cytoskeletal actin gene. These intron positions are identical to those of the analogous genes in mammals. These data suggest that vertebrate muscle and cytoskeletal actin genes became separate before the divergence of amphibians from the rest of the vertebrates. Other sequence studies performed include an examination of codon usage and sequence divergence in actin genes. 'Repetitive DNA' has been found in the 3.6 kb Eco Rl fragment and has been roughly localized. Primer extension has been used to show that the partial leader sequence of an oocyte mRNA species is identical to that found in the beta-like cytoskeletal actin clone. A large intron is present in the leader of this gene. Near the transcriptional start site, several small sequences have been found which exist in similar positions in the rat beta-cytoskeletal actin gene, and may possibly be involved in the control of gene expression. Another oocyte actin mRNA species encodes a protein similar to the N-terminal end of the mammalian gamma- cytoskeletal actin, and its leader sequence is surprisingly similar to that of the beta-like actin message. Primer extension has also been used to study the expression of actin genes in several laevis non-muscle tissues, as well as in JU borealis skeletal muscle.