Among-population differences in morphology and behaviors such as boldness have been shown to co-vary with ecological conditions, including predation regime. However, between- and within-population covariation of predator defense morphology with variation in behaviors relevant to ecology and evolution (boldness, exploration, activity, sociability and aggressiveness, often defined as personality traits when they are consistent across time and contexts) have never been quantified together in a single study in juvenile fish from populations found in contrasting environments. We measured predator defense morphology differences between adults from two freshwater populations of threespine sticklebacks with different ecological conditions. We then quantified five behaviors in juveniles from both populations raised in a common environment. Wild-caught adults showed significant differences in predator defense morphology. One population had significantly lower lateral plate number, shorter dorsal spine, pelvic spine and pelvic girdle. Furthermore, 61% of individuals from that population showed an absence of pelvic spine and girdle. At the population level, we found that differences in defense morphology in adults between the two lakes were coupled with differences in behaviors in juveniles raised in a common environment. Levels of activity, aggressiveness and boldness were higher in juveniles from the population lacking predator defense structures. At the individual level, anti-predator morphology of adult females could not predict their offspring’s behavior, but juvenile coloration predicted individual boldness in a population-specific manner. Our results suggest that ecological conditions, as reflected in adult predator defense morphology, also affect juvenile behavior in threespine sticklebacks, resulting in trait co-specialization, and that there is a genetic or epigenetic component to these behavioral differences [Current Zoology 58 (1): 53–65, 2012].