Inhibitory control (IC) is a dimension of child temperament that emerges in toddlerhood and involves the ability to regulate behavior in response to instructions or expectations. In general, children with low levels of IC have more cognitive and social difficulties, and higher levels of problem behaviors. Unfortunately, there is a paucity of research on the heritability of this important behavioral dimension. The present study used a twin design to examine the extent to which genetic and environmental factors contribute to individual differences in IC. Laboratory and parent assessments of IC were conducted on 294 same-sex twin pairs (133 MZ, 161 DZ) at 24 months of age. Model-fitting analyses showed that genetic factors accounted for 38 and 58% of the variance in laboratory- and parent-rated IC, respectively. Multivariate genetic analyses also revealed that the covariance between observed and parent-assessed IC could be predominantly explained by common genetic influences.