We have cloned a full-length cDNA for mouse cytoskeletal B-actin mRNA from poly (A)+ RNA of NIH3T3 cells and deduced the complete nucleotide sequence of the cDNA. This cDNA clone contains an entire coding frame for cytoskeletal B-actin, 5'untranslated sequence of 80 bp long and 3'untranslated sequence of 684 bp long. The nucleotide sequence of the coding region is extremely conserved among higher vertebrates. The 5'and the 3'untrans-lated regions show strong homologies with the corresponding regions of human (1-3) and rat (4) 6-actin mRNAs and weak homologies with chicken B-actin mRNA (5). This result confirms the evolutional conservation of the untrans-lated sequences of the 8-actin mRNAs among vertebrates, and suggests a func-tional importance of these regions possibly for the posttranscriptional control of the cytoskeletal 8-actin expression.(supported by the Grant-in-Aid for Cancer Research to SS from theMESC of Japan and by Grant-in Aid for Comprehensive 10-Year Strategy for Cancer Control to KT from the MHW, Japan.)