The Arp2/3 complex and filamins play important roles in organization of actin cytoskeleton, and thus in cellular morphology and locomotion. We recently identified decreased expression of a gene for one of seven subunits of the Arp2/3 complex, the p41-Arc gene, and silencing of a filamin gene, the FLNc gene, in human gastric cancers. In this study, gene expressions of the seven subunits of the Arp2/3 complex, including p41-Arc, and their methylation statuses were analyzed in human gastric cancers. Quantitative real-time RT-PCR analysis of 32 primary gastric cancer samples and eight gastric cancer cell lines revealed that expressions of all the seven genes were significantly decreased. All the 32 primary cancer samples showed decreased expression of at least one subunit, and 25 samples showed decreased expressions of four or more of the seven subunits. Methylation-specific PCR analysis showed that none of the CpG islands in the 5' regions of the six genes other than p41-Arc were methylated in primary gastric cancers or cell lines. The consistent decrease of the Arp2/3 complex genes and its important role in actin organization suggested that the decrease could be involved in cancer phenotypes, such as dysplastic morphology.