In SLA, it has been often assumed that the effect of the first language (L1) is not very strong in the acquisition of grammatical morphemes (e.g., Ellis, 1994; Mitchell & Myles, 2004). However, such an assumption has not been systematically examined in the literature. This article reviews the morpheme studies conducted with native speakers of Japanese, Korean, Chinese, and Spanish to test the effect of the L1 in the acquisition of grammatical morphemes. The review reveals that although Spanish L1 learners' acquisition order generally conforms to the "so-called" natural order (Krashen, 1977), native speakers of Japanese, Korean, and Chinese mostly acquire plural -"s" and articles later than, and possessive '"s" earlier than, is predicted by the natural order. This indicates that learners can acquire a grammatical morpheme later or earlier than predicted by the natural order, depending on the presence or absence of the equivalent category in their L1. This suggests that L1 transfer is much stronger than is portrayed in many SLA textbooks and that the role of L1 in morpheme acquisition must be reconsidered.