The present study examined the proposal that the presence of a similar morpheme in the learner's first and second languages (L2) facilitates morphological development in the L2. Advanced Russian and Japanese speakers of English as a second language performed a self-paced reading task in which they read English sentences word by word for comprehension. Russian participants showed a reliable sensitivity to plural errors, but Japanese participants did not. The findings supported the morphological congruency hypothesis. A theoretical proposal is put forward to explain how morphological congruency affects L2 morpheme acquisition. The findings and the proposal are relevant to the discussion of the critical period hypothesis, ultimate attainment in L2, and the characterization of L2 competence of steady-state adult L2 learners.