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The giant squid, Architeuthis dux, has been reported for centuries, but its distribution and ecology have remained famously mysterious. We investigated the A. dux distribution in the Sea of Japan using the environmental DNA (eDNA) method, which has recently been considered as a useful technique for evaluating the distributions of rare species. To develop the eDNA method to detect A. dux, we first created a species-specific polymerase chain reaction (PCR) primer-probe and tried to detect the eDNA in the western region of the Sea of Japan where A. dux has recently been observed. We successfully collected and amplified the A. dux eDNA in our study area using field water sampling and real-time PCR measurements. A. dux eDNA was detected in winter but not in summer, reflecting the historical record of A. dux observations in the region. The use of eDNA techniques could be a potential method for monitoring 'invisible' and rare organisms, even in open ocean habitats., Author(s): Toshifumi Wada [sup.1] [sup.2], Hideyuki Doi [sup.3], Daisuke Togaki [sup.4], Ryotaro Kaida [sup.3], Mariko Nagano [sup.3], Izumi Katano [sup.5], Masami Suzuki [sup.6], Tetsuya Ohtani [sup.6], Hiromune Mitsuhashi [sup.1] [sup.2] [...]