Fruit flies are recognised as one of the major pests of fruit and vegetable crops worldwide. Potential benefits from fruit fly research include biosecurity benefits from better quarantine surveillance that reduces the costs of an incursion by a damaging exotic pest fruit fly; market access benefits by enabling new fruit exports; and field control benefits from better crop management. The Australian Centre for International Agricultural Research (ACIAR)’s investment in fruitfly research goes back some 25 years to an initial project in Malaysia. Since that time, ACIAR’s continued investment has funded a total of 18 projects ranging across several areas of fruit-fly research, and covering Malaysia, Thailand, Philippines, Fiji Islands, Samoa, Tonga, Cook Islands, Vanuatu, Solomon Islands, Federated States of Micronesia (FSM), Papua New Guinea, Bhutan, Vietnam, Laos, and Indonesia. In an impact assessment study of all 18 ACIAR projects, Lindner and McLeod (2008) calculated that the present value (PV) of the total direct investment in these projects by ACIAR and its partners has been A$50.76 million. The PV of total quantifiable realised and prospective benefits that can be attributed to the direct investment by ACIAR and its partners was estimated to exceed A$258.84 million. Of this total PV of quantifiable benefits, A$212.63 million was calculated to accrue to partner-countries. In this paper, the question of why many potential benefits to partner-countries have not been realised to date, and why some future prospective benefits are problematic is examined. While the total value of benefits generated from the investment by ACIAR and its partners is impressive, the pattern of benefits is variable by type of benefit and by country. One of the most important general lessons, widely known but reinforced by the results from this study, is that while successful research project outcomes may be necessary to enable potential benefits, they rarely are sufficient for benefits to be realised. In particular, potential benefits will only be realised if there is uptake of project outputs. While it is recognized that the conditions for uptake are typically well beyond the influence of the researchers both in time and scope, at the time of project formulation, the necessary conditions for adoption of project outputs often seem to receive insufficient attention. Notwithstanding some 20 years of research on the development of low-cost protein bait sprays from brewery waste, the benefits are still essentially prospective and it has not been conclusively demonstrated that the use of these sprays will be widely adopted as a cost-effective alternative to existing practices in developing countries.