Life-history evolution in short-lived organisms has been investigated extensively, but little is known empirically about the causes of the different life-history strategies of long-lived organisms. To explore this issue, we conducted experiments to evaluate natural selection acting on key traits during an important life-history stage of the red-eared slider turtle (Trachemys scripta elegans). We reared eggs from 56 females in a seminatural common environment and released 356 of the resulting hatchlings in four replicates at a natural nesting area to assess offspring recapture probability as a measure of survivorship during the post-emergence migration. Larger body size of hatchling turtles was strongly favored by natural selection overall (β′ = 0.346, P < 0.001) and in each replicate. Although selection on body size was positive in all analyses, the form of selection varied depending on the inclusion of nonrecaptured presumed dead individuals in the statistical analyses. This result indicates that such analytical tools are sensitive to the common assumption in many mark–recapture studies that nonrecaptured individuals have no fitness (i.e., W = 0). The most likely explanation for the size-dependent recapture probabilities is differential mortality, because body size at release for survivors was significantly larger than body size at release for hatchlings found dead. This pattern of survivorship appeared to result from size-dependent predation. We suggest that the benefit of larger body size for hatchling turtles may reside in improved locomotor performance that reduces the duration of exposure of larger individuals to predation. This hypothesis is supported by significant negative correlations between measures of body size and time elapsed between release and recapture.