This study analysed children's use of mental computation strategies and standard written algorithms in the domain of multi-digit addition and subtraction, using the choice/no-choice method. Twenty-one Flemish fourth-graders (M[subscript Age] =9y10m) solved problem-items that either stimulated the use of mental computation strategies or a standard written algorithm, in one choice and two no-choice conditions. In the choice condition, children could use their preferred strategy on each item. In the two no-choice conditions, they had to solve all items with, respectively, a mental computation strategy (no-choice mental condition) and the standard written algorithm (no-choice written condition). The results revealed that children tended to use the written algorithm on multi-digit sums, even on sums that evoke a mental computation strategy. Moreover, they also used the standard written algorithms with high efficiency and flexibility on sums that elicit a mental computation strategy. We discuss the theoretical, instructional and methodological implications of these results.