The primary objective of my dissertation research was to: 1) develop tests that measure personality in guppies, 2) experimentally validate these tests, and 3) investigate and compare the phenotypic and genetic relationships between ecological factors and personality traits across developmental phases and across populations. I explored the interrelationships of suites of behaviors within individuals in two guppy populations representative of high- and low-predation regimes. I compared variation on behavioral reactions of subjects with known genetic similarity across four important ecological settings - Exploration of an unfamiliar environment, Foraging on a novel food item, Social behavior, and Boldness under threat of predation. Subjects were tested as juveniles and adults in each setting (Chapter 2). Behavioral dimensions were highly plastic over developmental time for low-predation populations. One out of four behavioral dimensions were phenotypically stable over developmental time in individuals from the high predation population. I found no significant differences between populations in behavioral factors (Chapter 3). Additive genetic variances for single traits ranged from zero to one. Genetic correlations between traits were largely non-significant although a significant positive genetic correlation occurred between Sociability and Boldness in juveniles from the low-predation population. Populations also differed in the extent to which additive genetic effects contributed to variation in behavior. There was less overall genetic diversity among individuals from the high-predation population. Finally, there was little evidence for behavioral syndromes (positive genetic correlations) in either population (Chapter 4).