This paper examines the history of, and current practices associated with, “mass ecotourism” in the long-established resort destinations of Phuket, Thailand and Bali, Indonesia. Using examples from each island’s original mass ecotourism companies, this paper argues that, contrary to approaches that tend to universalize the concept, ecotourism remains highly anchored to local or regional context. In the case of resort areas of Southeast Asia, it derives its origins and ongoing success to fundamental links to conventional mass tourism. A symbiotic relationship between mass tourism and ecotourism has allowed companies in Phuket and Bali to promote such ecotourism principles as conservation, ethical management, and environmental education by tapping into the markets, marketing channels, and business networks of conventional mass tourism. This mass-eco synthesis, coupled with other historical and structural parallels between mass ecotourism in Phuket and Bali, characterizes a Southeast Asian model of ecotourism centered on established resort destinations.