Different types of actin-related proteins which constitute an actin-superfamily together with conventional actin have recently been described (Mullins et al., 1996). Among them, Arp1 exhibits the highest homology with conventional actin. With the aim of clarifying the cellular function of Arp1 in mammalian cells, we cloned the cDNA encoding mouse α-Arp1, one of the variants of Arp1, from a mouse diaphragm cDNA library; two types of α-Arp1 cDNAs, which are probably generated by alternative RNA splicing from a single gene, were obtained and the entire sequences were determined. They differed only in the presence or absence of an insertion of 1.3 kb in the 3'-non-coding region but shared a common open reading frame. The deduced amino acid sequence was identical with that of human α-Arp1. Northern blot analysis showed that the a-Arp1 mRNA corresponding to the longer cDNA is transcribed not only in various non-muscle tissues but also in muscle tissues, while the transcript corresponding to the shorter one becomes expressed only in skeletal muscle as development progresses. It is suggested that α-Arp1 may play some role in muscle, as judged by the significant level of its expression.