Working memory capacity (WMC) has received attention across many areas of psychology, in part because of its relationship with intelligence. The mechanism underlying the relationship is unknown, but the nature of typical WMC tasks has led to two hypothesized mechanisms: secondary-memory processes (e.g., search and retrieval) and the maintenance of information in the face of distraction. In the present study, participants (N= 383) completed a battery of cognitive tasks assessing processing speed, primary memory, working memory, secondary memory, and fluid intelligence. Secondary memory was the strongest predictor of fluid intelligence and added unique predictive value in models that accounted for working memory. In contrast, after accounting for the variance in fluid intelligence associated with the secondary-memory construct, the working memory construct did not significantly predict variability in fluid intelligence. Therefore, the secondary-memory requirements shared by many memory tasks may be responsible for the relationship between WMC and fluid intelligence, making the relationship less unique than is often supposed.