High- and low-task-importance Ss read a strong or weak unambiguous message or an ambiguous message that was attributed to a high- or low-credibility source. Under low task importance, heuristic processing of the credibility cue was the sole determinant of Ss' attitudes, regardless of argument ambiguity or strength. When task importance was high and message content was unambiguous, systematic processing alone determined attitudes when this content contradicted the validity of the credibility heuristic; when message content did not contradict this heuristic, systematic and heuristic processing determined attitudes independently. Finally, when task importance was high and message content was ambiguous, heuristic and systematic processing again both influenced attitudes. Yet, source credibility affected persuasion partly through its impact on the valence of systematic processing, confirming that heuristic processing can bias systematic processing when evidence is ambiguous. Implications for persuasion and other social judgment phenomena are discussed.