(1) In seepage-filled borrow-pits in South India it was observed that there was a progressive decline in the density of larvae of Anopheles culicifacies, Giles, as the pits became older. The largest numbers of larvae were tound soon after water entered the newly-dug pits.
(2) There was less ovipositing by culicifacies in older pits than in new ones dug late in the irrigation season. Newer pits seemed definitely more attractive in this species than older ones. These newer pits sheltered more culicifacies larvae late in the season than the older pits.
(3) The decline of culicifacies larva density in a borrow-pit seemed to be due mainly to factors internal to the pits. There was no evidence of the influence of external factors, except from October to January, when perhaps meteorological influences supplemented the internal factors. The attractiveness of new borrow-pits to culicifacies appeared to be due mainly to internal factors.
(4) Certain simple factors studied did not seem to have any significance in relation to culicifacies density in the pits. Rainfall, predators, macroscopic vegetation, pH, CO2, dissolved oxygen, bicarbonate alkalinity, ratio of free to bound and half bound CO2, hardness, chlorine, ammoniacal nitrogen, nitrates, nitrites, sulfates and iron, appeared to have no significance in this regard. Albuminoid nitrogen and oxygen absorbed perhaps had some significance, which was not clear.
(5) Among planktonic organisms, the individual groups of organisms, such as green algae, diatoms, rotifers, and copepods, definitely showed no relation to culicifacies breeding. Protozoa as a group appeared to be negatively associated to a slight degree. Blue-green algae also seemed to have a negative association.
(6) Amorphous organic matter and total plankton, however, showed statistically significant negative association with larval density of culicifacies The decline in culicifacies larvae was clearly associated with increase in total plankton and amorphous matter. The attractiveness of new borrow-pits also seemed to be related to their low total plankton content.
(7) The exact manner in which the total organic matter acted as an inhibitory factor against culicifacies breeding was not determined.