Acidic extracellular pH is a hallmark phenotype of solid tumor. We have shown that acidic pH induced matrix metalloproteinase-9 (MMP-9) expression in B16 melanoma cells through Ca2+-triggered phopholipase D - MAP kinases - NFκB signaling. Here, we investigated whether acidic extracellular pH is a candidate microenvironment to induce epithelial-mesenchymal transition (EMT) using the Lewis lung carcinoma (LLC) model. First, we established two LLC variants with differential metastatic ability by repeated tail vein injection of tumor cells from metastasized foci in lung, named LLC-meta-I (low metastatic) and LLC-meta-IV (high metastatic). Morphology of LLC-meta-I and LLC-meta-IV cells was a cobble-stone like and fibroblastic, respectively. When LLC-meta-I cells were cultured with acidic pH medium, cell shape became fibroblastic which was similar to LLC-meta-IV. Then, we checked whether matrix metalloproteinase-9 (MMP-9) production was induced by acidic pH. In good accordance with the B16 melanoma model, MMP-9 was extremely induced in LLC-meta-IV at acidic pH as compared with LLC-meta-I cells. This was also confirmed by RT-qPCR technology. RT-qPCR analysis showed that acidic pH induced vimentin expression but reduced E-cadherin expression, which are typical features of EMT. These data suggested acidic extracellular pH is a microenvironmental niche to induce EMT in some kinds of cancer.