An actin-encoding cDNA of the kuruma shrimp (Marsupenaeus japonicus) was cloned from a hemocyte cDNA library. To obtain the full length of actin cDNA, a partial cDNA fragment of 517 bp was isolated by reverse transcriptase-polymerase chain reaction (RT-PCR) from poly (A) RNA derived from hemocytes and used as a probe for plaque hybridization. The full actin cDNA was 1,327 bp in length, encoding 376 amino acid residues. The deduced amino acid sequence had 100% identity to that of β-actin from the Pacific white shrimp Litopenaeus vannamei, and extremely high similarity to the sequences of actins in other vertebrates and invertebrates. The cDNA fragments of the actin were amplified by RT-PCR from all organs tested: lymphoid organ, stomach, hepatopancreas, heart, kidney, muscle, eye, ovary, and testis.