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We document and interpret microgeographic variation in life history traits of water pythons (Liasis fuscus) on the Adelaide River floodplain in tropical Australia. Subpopulations of pythons separated by <2 km differ in reproductive timing, in survival rates at three different life history stages (adults, embryos, and hatchlings), in costs of reproduction, and in reproductive frequencies. Remarkably, these differences all seem to result from a minor divergence in nest-site characteristics, Female water pythons use two main types of nest sites: those with relatively low, variable temperatures (especially hollows within paperbark root systems on the edge of the floodplain) and those with higher, constant temperatures (burrows dug by large varanid lizards in the higher, drier ridges similar to 2 km away). "Cool" nests delayed reproduction and reduced survival rates of hatchlings in at least one year. Females ovipositing in "cool" nests remained with the clutch throughout the 2-mo incubation period, whereas they deserted the clutch within a few days of laying in warmer nests. Most egg-attending females did not feed. Hence, "cool"-nesting females were emaciated by the end of incubation, and many died from starvation or predation; surviving females required two years to replenish their energy reserves before producing another clutch. In contrast, "hot"-nesting females had higher rates of survival, and most could reproduce again the following year. Most females showed strong nest-site fidelity in successive clutches, but some moved between "hot" and "cool" nest sites. We have previously shown that the incubation thermal regime also affects developmental rates and hatchling phenotypes (shape and behavior). Thus, nest-site selection is a phenotypically plastic character, which although superficially trivial, engenders significant microgeographic variation in a wide range of life history traits.