Koyagawa and Otozuregawa Firefly Habitat

Kiyagawa and Otozuregawa Genji Firefly Habitats: An Enchanted Summer Night in Yamaguchi

On warm, windless evenings in June, two rivers in the mountains of western Yamaguchi Prefecture transform into rivers of light. Tens of thousands of Genji fireflies (Luciola cruciata) rise from the water's edge, blinking in unison as they drift over the current. Designated as a National Natural Monument in 1958, the Kiyagawa and Otozuregawa Genji Firefly Habitats are among Japan's most celebrated firefly viewing destinations—a place where you can witness one of the country's most ancient natural spectacles while enjoying centuries-old hot springs and traditional boat rides.

What Are the Kiyagawa and Otozuregawa Firefly Habitats?

The designated area spans two separate river systems flowing through Shimonoseki City (Toyota Town) and Nagato City in northwestern Yamaguchi Prefecture. The Kiyagawa River rises near Mount Osasa-toge in Tawarayama and winds south through Toyoda Lake before reaching the Suo Sea. The Otozuregawa River (also called the Fukawa River) flows through Nagato Yumoto Onsen, Yamaguchi's oldest hot spring resort, and out to the Sea of Japan.

Although both rivers host the same species of Genji firefly—Japan's largest firefly, with a body length of 12 to 18 millimeters—the timing of their emergence differs. Because the Otozuregawa basin faces the Sea of Japan and has slightly cooler temperatures, its fireflies appear about ten days later than those at the Kiyagawa. The peak at the Kiyagawa typically falls around June 10, while the Otozuregawa reaches its height around June 20.

Why This Habitat Was Designated a National Natural Monument

The fireflies of the Kiyagawa have long been affectionately known as "Toyoda Hotaru" (Toyoda Fireflies), while the fireflies dancing along the Otozuregawa have been a celebrated symbol of Yumoto Onsen for centuries. In the early twentieth century, however, both populations declined sharply due to overcollection by tourists and commercial dealers, who harvested the insects for sale.

To protect these precious habitats, the national government designated the area as a Natural Monument on June 11, 1958, under Notification No. 49 of the Committee for the Protection of Cultural Properties. The designation covers an exceptionally wide area, including not only the main rivers but also their tributaries—the Nanaegawa, Yasudagawa, Kurokawagawa, and Kizugawa branches feeding into the upper Kiyagawa, as well as the Ōkōchigawa and Daineijigawa flowing into the Otozuregawa. Local elementary and junior high schools throughout the designated area now participate in firefly breeding and habitat research, ensuring that the tradition is passed on to future generations.

Highlights and How to Enjoy the Fireflies

The Hotarubune: Japan's First Firefly Boat

The undisputed highlight of any visit is the Hotarubune (Firefly Boat) on the Kiyagawa, a traditional Japanese poling boat that drifts approximately 800 meters down the river under the canopy of fireflies. Launched in 1991 by local volunteers, this was the first firefly-viewing boat operation in Japan and has since become an iconic summer tradition of Toyota Town. As the boat glides gently along the dark water, thousands of tiny lights blink in synchrony around you—a scene often compared to a living Christmas tree of stars suspended above the river. The boats run from early to late June; reservations through the Toyotacho Tourism Association are essential, as weekend evenings fill up months in advance.

Yumoto Onsen: Fireflies Above the Hot Spring Town

At Nagato Yumoto Onsen, the fireflies have a different character. Here you can enjoy them while soaking in the open-air bath of a riverside inn, strolling the stone-paved promenades, or sitting on one of the riverside terraces (kawadoko) built directly above the Otozuregawa. The historic Banjaku Bridge, dating from the early Edo period, frames the scene beautifully. The town was extensively renewed in 2020 under the theme "Heaven of the Great Outdoors" (Osoto Tengoku), and the soft, warm illumination along the river creates a magical atmosphere where firefly light and human light blend together.

Toyota Hotaru no Sato Museum

Right beside the Kiyagawa stands a striking silver dome-shaped building modeled after a Genji firefly itself. The Toyota Hotaru no Sato Museum opened in 2004 and has served as the Shimonoseki Municipal Museum of Natural History since 2020. Its seven exhibition zones—from the immersive "Welcome to the World of Genji Fireflies" gallery to live aquarium displays of firefly larvae—let you learn about firefly ecology in any season. A fascinating fact you might discover here: fireflies west of the Fossa Magna geological line (including those of Yamaguchi) flash every two seconds, while those east of it flash every four seconds.

Surrounding Attractions

The firefly habitats sit at the heart of one of Yamaguchi's most beautiful regions, with many other destinations worth exploring.

• Nagato Yumoto Onsen: Yamaguchi's oldest hot spring, with about 600 years of history. Said to have been discovered by the priest Jōan of Daineiji Temple following a divine revelation, its alkaline simple thermal water is celebrated for smoothing the skin.
• Daineiji Temple: An ancient Sōtō Zen temple just five minutes by car from Yumoto Onsen, renowned for spectacular autumn maple leaves.
• Motonosumi Shrine: About 30 minutes by car from Yumoto, this seaside shrine is famous for its 123 vermilion torii gates descending dramatically toward the Sea of Japan.
• Akiyoshidai Plateau and Akiyoshido Cave: Japan's largest karst plateau and East Asia's largest limestone cave, about 40 minutes by car.
• Tsunoshima Bridge: A famously photogenic bridge stretching across emerald-blue waters to Tsunoshima Island in Shimonoseki.
• Michi-no-Eki Hotaru Kaido Nishinoichi: A roadside station next to the museum, with a hot-spring bathhouse ("Hotaru no Yu"), local restaurants, and shops selling regional produce—the meeting point for Firefly Boat rides.