Recent studies hypothesize that language production is governed by the principle of efficient information transmission: speakers tend to omit elements whose information content is contextually predictable while providing more linguistic signal to convey otherwise less predictable information. However, previous findings in support of this hypothesis are also compatible with alternative accounts based on production difficulty. To distinguish between these competing accounts, we conducted experiments on speaker’s preference in optional casemarking in Japanese. The results suggest that Japanese speakers are more likely to omit the object case-marker when an associated noun has semantic properties that are prototypical to a grammatical object. Moreover, case-marker omission was facilitated when other elements in a sentence made the grammatical function assignment more predictable. The results were obtained with all the factors related to production difficulty held constant, and thus support the models of communicatively efficient language production.