Morphological complexity (MC) is a relatively new construct in second language acquisition (SLA). After critically discussing existing approaches to calculating MC in first- and second-language acquisition research, this article presents a new operationalization of the construct, the Morphological Complexity Index (MCI). The MCI is applied in two case studies based on argumentative written texts produced by native and non-native speakers of Italian and English. Study 1 shows that morphological complexity varies between native and non-native speakers of Italian, and that it is significantly lower in learners with lower proficiency levels. The MCI is strongly correlated to proficiency, measured with a C-test, and also shows significant correlations with other measures of linguistic complexity, such as lexical diversity and sentence length. Quite a different picture emerges from Study 2, on advanced English learners. Here, morphological complexity remains constant across natives and non-natives, and is not significantly correlated to other text complexity measures. These results point to the fact that morphological complexity in texts is a function of speakers' proficiency and the specific language under investigation; for some linguistic systems with a relatively simple inflectional morphology, such as English, learners will soon reach a threshold level after which inflectional diversity remains constant.