Fifty 8-month-old infants participated in a study of the interrelations among cognition, temperament, and electrophysiology. Better performance on a working memory task (assessed using a looking version of the A-not-B task) was associated with increases in frontal–parietal EEG coherence from baseline to task, as well as elevated levels of frontal–occipital coherence during both baseline and task. Enhanced performance was also associated with decreased heart period (increased heart rate) from baseline to task. Infants with better working memory performance had parents who rated them high on activity level and distress to limitations. When considered collectively, EEG coherence and heart period contributed unique variance in the prediction of high and low performance groups. Implications for the study of infant cognition are discussed.