Summary Delays in malaria eradication programs are caused more by non-response of fully susceptible vectors to attack measures than by physiological resistance, though the latter receives more attention. Currently recommended methods for vector studies are reviewed, with suggestions for their extension. Three principal South American vectors are shown to rest mainly outside houses by day; some exophily existed before the use of DDT. Their biting cycles are unimodal, with peak activity near midnight, contrasting with minor vectors and non-vectors with bimodal or crepuscular cycles. A. nuneztovari, hitherto thought an anomalous exophagic and crepscular vector, includes two species, a widely distributed non-vector, A. dunhami, and an endophagic nocturnal vector. Observations, mainly from Colombia, on nocturnal contacts with indoor surfaces by vectors, and on entry to and exit from houses are described; contacts were mainly with the lowest parts of walls, well below the height to which they are sprayed. The times of sighting of vectors resting unfed and fed showed normal distributions, their means separated by intervals of 30 to 150 minutes. In A. darlingi, the interval was 150 minutes in unsprayed, 78 minutes in sprayed houses; in A. punctimacula and A. nuneztovari in sprayed houses, the intervals were 96 and 30 to 60 minutes, respectively. The durations of the contacts showed skewed distributions with short periods, medians 1 to 2.5 minutes in all three species, predominating. Mean durations, biased by a few long periods, were greater, for A. darlingi reaching 12 minutes in sprayed houses, 6 in unsprayed. Times of entry and exit to houses, studied in A. darlingi with a drop-net lowered to enclose a house at intervals through the night, showed that, before spraying, about half the mosquitoes entering fed inside, during a stay of about 2 hours. For 3 months after use of a DDT-HCH mixture, 65% of those entering failed to leave; mean stay was 3 hours. Later, with DDT treatment only, the proportion entering and failing to leave fell to 28%. Vector attack, human receptivity, and infectivity of the plasmodial gametocyte, all reach maxima inside houses, late at night; this is the key to persistence of transmission by susceptible vectors. In spite of the prevailing daytime exophily, vectors make multiple short contacts with insecticide-treated surfaces, repeated on each gonotrophic cycle; these are inadeuate to halt transmission in areas of low elevation, high humidity, and temperature, occupied by poorly housed colonists; insecticides lacking the locomotor stimulant action of DDT may be the remedy.