The transgenic RIDL method could ensure that male mosquitos can be released without biting females and that the males would have no female progeny after mating to wild females. Urban Anopheles or Aedes vector populations, surrounded by different species in rural areas, would seem to be the most appropriate targets for such releases, aiming at eradication. In urban areas intensity of transmission is generally not very high and the public-health outcomes of such urban programmes could be monitored by passive surveillance through health facilities or by active surveillance for infections with or without associated symptoms. The alternative use of transgenic mosquitos would be to produce strains refractory to infection by pathogens such as Plasmodium and to drive such genes into wild populations. In theory, in contrast to sterile-male eradication, such a procedure could "resist" a limited level of immigration and could open up the possibility of using the method against African rural malaria. However, in practice it would seem extremely difficult or impossible to ensure the necessary complete linkage of the refractoriness genes to the driving system. If this problem could be overcome one could monitor the impact of the spreading of the refractoriness genes by its impact on (i) the sporozoite rate in the wild population; (ii) the incidence of re-infection after clearing existing infections with an appropriate drug treatment; (iii) active surveillance for prevalence of malaria fever and anaemia in children; (iv) attacks of severe malaria and deaths monitored though hospitals and village reporters.