La Selva (tel. 506/2766-6565, www.ots.ac.cr ), four kilometers south of Puerto Viejo , is a biological research station run by the Organization of Tropical Studies (OTS). The station is centered on a 1,500-hectare reserve—mostly premontane rainforest but with varied habitats—linked to the northern extension of Braulio Carrillo National Park .
More than 420 bird species have been identified here, as have more than 500 species of butterflies, 120 species of mammals, and 55 species of snakes. The arboretum displays more than 1,000 tree species.
Almost 60 kilometers of trails snake through the reserve. Some have boardwalks; others are no more than muddy pathways. Rubber boots or waterproof hiking boots are essential, as is raingear.
Guided nature walks are offered at 8 a.m. and 1:30 p.m. daily ($30 adults, $22 children half day; $38 adults, $28 children full day), and an early-bird bird-watching tour departs at 5:30 a.m. You may not explore alone.
Only 65 people at a time are allowed in the reserve, including scientists. It is often booked solid months in advance. Reservations are required.
The OTS operates a shuttle van from San José  on Monday ($10), space permitting (researchers and students have priority), and between La Selva and Puerto Viejo  Monday–Saturday. Transporte Caribe (tel. 506/2221-7990) buses from San José will drop you off at the entrance, from where you’ll need to walk to La Selva (2 km).