The authors examined whether individual differences in fluid intelligence (gF) modulate the testing effect. Participants studied Swahili--English word pairs and repeatedly studied half the pairs or attempted retrieval, with feedback, for the remaining half. Word pairs were easy or difficult to learn. Overall, participants showed a benefit of testing over restudy. However, almost 1/3 of the sample had a negative testing effect and benefitted more from restudy than testing, as well as performing better overall. These individuals self-reported less use of shallower encoding strategies than positive testing effect participants but did not differ in other dimensions. For individuals with a positive testing effect, difficulty had differential effects on participants who scored high or low on a measure of gF, with high gF participants showing larger testing effects for difficult over easy items, whereas low gF participants showed the opposite. Working memory performance was not related to the magnitude of the testing effect; however, vocabulary knowledge revealed a similar pattern as gF, with higher vocabulary associated with a testing effect for difficult but not easy items. This suggests that the benefit of retrieval practice varies with item difficulty and participant abilities. Thus, recommendations to engage in retrieval practice should take into consideration the interactive effects of to-be-learned materials and individual differences in the learners.