Fears and phobias are relatively common in childhood. Both environmental and genetic theories have tried to explain the etiology behind these conditions. However, data supporting the different theories are sparse. To investigate the relative importance of genetic and environmental influences on specific phobias and fears, parental reports of animal, situational, and mutilation fears and phobias were completed for 1106 pairs of 8- to 9-year-old Swedish twins. The prevalence of specific phobias was 7.3% for boys and 10.0% for girls. Genetic, shared environmental, and nonshared environmental effects contributed to individual differences in fears and phobias in young children, but the magnitude of the effects differed between sexes. Shared environmental effects contributed to a general susceptibility for fearfulness. Genetic and nonshared environmental effects, on the other hand, contributed both to the general susceptibility and specific fearfulness, even though these effects primarily were fear specific. These results indicate that both heritable factors as well as environmental factors such as trauma, vicarious learning, and/or negative information are important for differences in fearfulness and phobias—at least in children.