Few experimental studies provide data on early comprehension of negation. Commonly accepted norms are based on parental report and observational studies using a small number of participants. The purpose of this study was to determine if 18-month-olds (n=24) understand the word not. The study used a preferential looking paradigm, in which children saw two video screens showing a puppet performing a different action in each video. They then heard a voice, telling them to "Look! The ____'s not ____ing." For the three sets of videos used in the study, children only looked significantly longer at the matching video during one set of trials. However, for no set of trials did the children look longer at the puppet overtly named in the auditory stimulus. These results suggests that although children did not demonstrate a clear understanding of the word not, they may be developing an understanding of not at this age.