1. (1) An investigation was carried out in a hyperendemic malarious village in the Gold Coast to study the factors determining the true reservoir of falciparum malaria infection in the human population. 2. (2) A falciparum gametocyte (crescent) survey was carried out on the, population, and the actual infectivity of as many crescent carriers as possible was tested by means of feeds with laboratory-bred A. gambiae. 3. (3) My previous findings in Jamaica and Young's in South Carolina were confirmed in showing that some carriers are still infective to Anopheles at a time when crescents are so scanty that they can hardly be detected in the thick blood film.In this survey these cryptic carriers formed about 25 per cent. of all infective carriers. Nearly half of all infective carriers were infective at crescent densities lower than 20 per c.mm. 4. (4) About 10 per cent. of crescent carriers were non-infective at a time when crescents were abundant in the blood. The highest proportion of these non-infective carriers was in the infant group. 5. (5) The incidence of good infectors was highest in the pre-school children (toddlers) and school children (juveniles), the latter group, aged,5–9 years, being probably more important as a source of infection. 6. (6) Despite the low gametocyte rates and gametocyte density in adults, random test feeds revealed two out of 30, i.e. 7 per cent., infective. For this and other reasons the adult group may be a more important source of infection than usually considered under hyperendemic conditions. 7. (7) The rôle of these infectors is discussed in the light of recent knowledge about the uneven distribution of anopheline bites and feeds among different age-groups. 8. (8) Preliminary study on the reservoir of bancroftian filariasis showed, in random tests, about one-third of the adult population infective. The youngest infector was aged about 5, and the oldest about 75. Most infectors had a very low grade of infectivity. 9. (9) Below 100 microfilariae per 20 c.mm. there is no obvious relation between microfilarial density and the proportion of the mosquito batch which becomes infected and infective. Above 100 microfilariae per 20 c.mm., increased microfilarial density tends to produce a high infection rate, but there is no corresponding increase in the intensity of each infection. 10. (10) Preliminary studies on the nocturnal infectivity of human infectors to A. gambiae suggests that a fairly steady degree of infectivity is maintained from about 9 p.m. till about 4 a.m. 11. (11) In dissections of the mosquitoes caught in the village, the oocyst rate of A. gambiae was a little over four times that of A. funestus, but the sporozoite rate was 30 times greater. Similarly, despite an almost equally high rate of infection with immature filaria larvae, A. funestus had a very much lower rate of infection with mature filaria larvae than A. gambiae. 12. (12) A method is described of interpreting the mosquito infection rates in such a way as to throw some light on the fluctuations in infectivity of the human population as a whole.