Many known miRNAs in fish come from zebrafish and fugu whose genome sequence data are available. The Japanese flounder undergoes typical metamorphosis which is characterized by major morphological, functional, and behavioral changes during growth due to this metamorphosis from larva to juvenile. Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Here, the high-throughput sequencing was adopted to identify the miRNAs during metamorphosis in the Japanese flounder. We found abundant microRNAs during metamorphosis in the Japanese flounder. Overall design: Small RNAs were sequenced from metamorphosis stages of Japanese flounder