The current study pursues two goals. First, it establishes developmental trajectories in the acquisition
of 10 morphosyntactic features of Russian by American learners, using a grammaticality judgment
task (GJT), an offline test of morphosyntactic knowledge that allows for direct comparison of native
and nonnative performance through a highly controlled set of materials. Second, it compares the
performance of late second language learners and heritage speakers (early learners) of Russian
matched in global proficiency as established by the ACTFL Oral Proficiency Interview (OPI), and
ranging from Intermediate to Superior proficiency. The study demonstrates that heritage speakers
outperform late second language learners on most, but not all the morphosyntactic features tested in
the GJT. These findings shed new light on the development of nonnative grammatical knowledge in
early and late learners of Russian, and will inform Russian language curriculum development.