The purpose of the present study was to determine whether temperament differences exist between infants who completed a visual perceptual/cognitive experiment and those who did not. A total of 14 Caucasian infants ranging in age from 5-15 months participated in the study. The subjects were placed in one of two groups (completers vs. noncompleters) based upon their performance at four months of age in a visual habituation experiment. Infants were classified as noncompleters if they had fussed, cried or fallen asleep two or more times during the test session and could not be coaxed into a more attentive state (i.e., to be visually alert, motorically inactive). Additional information on the infants' behavior patterns was obtained from their mothers who were asked to complete the Infant Temperament Questionnaire Revised (ITQR). Based upon personal observation and parental comments from previous investigations it was predicted that the subjects in the completers group would tend to be classified as easy or intermediate in temperament whereas the noncompleters group would generally be labeled as difficult or slow to warm up. In general, results supported this prediction. In comparison to the completers, the noncompleters were found to be more motorically active, withdrawing, and negative in mood. The findings were discussed in terms of the questionable external validity of most infant perceptual/cognitive investigations. (Author/MP)