As the biomass of Aedes aegypti (L.) larvae increased in relation to the volume of rearing waters, oviposition attraction of these waters to conspecific, gravid females first rose to a peak and then declined. Further increases in biomass rendered waters strongly repellent. Comparable responses were elicited by a decrease in the volume of rearing waters or an increase in the relative size or number of mosquito larvae. Low volumes of water reduced oviposition attraction and increased repellency, whereas larger volumes increased attraction. Excessively large volumes diluted attraction to neutrality. Constraints imposed by the physical dimensions of the larval environment which interfered with the normal postural movements and behaviors of the larvae also induced repellency, independent of rearing volume. Titration of repellent waters revealed that infection with the digenean Plagiorchis elegans (Rudolphi) generated the most powerful repellent effect, whereas crowding or starvation induced significantly weaker responses. At no time did dilution of repellent waters restore attractive properties. Repellents, even at minute concentrations, overrode attractants. The density-dependent action of oviposition attractants and repellents may help to maintain larval populations near optimal levels through their influence on recruitment. Oviposition repellency induced by sublethal infections with P. elegans may maintain population levels below the carrying capacity of the environment. Persistence of oviposition attraction and repellency varied inversely with temperature. The magnitude of the repellent effect induced by larvae appears to be a good indicator of their probability of survival and may be of selective significance. Repellency may deflect ovipositing females away from sites close to human habitation and may lead to new, and perhaps more effective, method, to control mosquitoes and the diseases they transmit.