Study of a population of Wyeomyia smithii in western New York showed that regulation (steadiness) of density was most marked in the larval part of the life cycle. Sampling and experimental data from the field as well as laboratory experimental data indicated that larval density might be regulated to densities below the carrying capacity of the larval environment at least part of the time. The responses of larval cohorts to different food and larval density combinations in a laboratory experiment under a spring or late summer temperature and photoperiod regime showed that: regulation of numbers and biomass within larval populations in the early stages of development was more by variation in larval growth and unequal sharing of food than by mortality; 50-75% of the larvae survived for two months even under extreme food stress; each larva made a decision between pupation and diapause in the 3rd instar between days 20-30 of life; the food level widely modified the fraction of the cohort entering diapause (from 15-100% of cohort diapausing) around the expectation from photoperiod alone (56% diapausing). When individuals did complete the life cycle, they realized a positive average fitness, or intrinsic rate of increase. At food levels where fitness would be expected to have a negative average, all surviving individuals entered diapause. Seasonal switching of the direction of natural selection for and against the tendency to diapause, as opposed to continuous selection for rapid development with no diapause, is suggested as the mechanism which holds much genetic variation for both developmental rate and the tendency to diapause in a natural population of W. smithii. This approximates Roughgarden's (1971) model for density-dependent selection and maintenance of a polymorphism by seasonality. The range of fertilities expressed by W. smithii, a species which has obligate autogeny, matches rather closely the fertilities of other species of mosquitoes with some pronounced degree of autogeny. Reduced fertility, and by inference reduced innate fitness, may always accompany autogeny. On empirical grounds, it is suggested that an average of 80-100 eggs per female is the upper range for pure autogeny and if fertility falls below an average of 25 eggs per autogenic female, a population cannot persist without taking blood. Within these fertility boundaries some mosquito species have evolved, or are evolving, simplified autogenic life cycles.