Life-history theory posits causal links from an organism's phenotype (e.g., its body size and condition) to its reproductive output (e.g., offspring size and number) and thence to its microevolutionary fitness. However, few empirical data are available to quantify such links, especially for tropical organisms. Oviposition by field-caught keelback snakes (Tropidonophis mairii, Colubridae) in tropical Australia yielded data on clutches from 318 adult females (3151 eggs) over three years, and mark-recapture studies (1804 nights surveyed) quantified survival rates of neonates from those laboratory-incubated clutches. Path analysis revealed strong links between maternal phenotype, reproductive output, and fitness, and these links were consistent among years and across different parts of the nesting period. Although all eggs were incubated at standard water potentials, clutches varied considerably in the amount of water uptake by eggs during incubation. Hatchling body length was influenced more by the egg's water uptake than by its initial mass. Larger offspring were more likely to survive their first year of life. Overall, the rate of water uptake by a female's eggs was more important for her reproductive success (number of surviving progeny) than were variables such as mean, egg mass. Hence, this population experiences intense selection not only on "conventional" life-history traits such as egg size and number, but also on maternal nest-site choice and on the physiological mechanisms that control water exchange between the egg and the nest substrate.