Participants were exposed to 3 conditions in a between-groups design. Participants were told the experiment was about extrasensory perception and were asked to select the word from a pair of words that they thought the experimenter was thinking about. In 1 condition selections of the word with a double letter were reinforced with positive verbal feedback, and in another condition selections of nondouble letters were reinforced. The word selections of a control group were reinforced according to a random and predetermined order. By the end of the experimental session, participants who were reinforced for selecting a particular type of word, double letter or nondouble letter, selected that word significantly more than the alternative that was not associated with reinforcement and those participants in the control group selected the double-letter and nondouble-letter words equally often. In a postexperimental questionnaire none of the participants reported an awareness of the contingency. The present experiment replicates and extends previous work and confirms that participants' behavior can be modified by reinforcement without the participants' conscious awareness of the contingency.