This study was designed to investigate the development of third-person singular (3SG) -"s" in children who learn English as a second language (L2). Adopting the usage-based perspective on the learning of inflection, we analyzed spontaneous speech samples collected from 15 English L2 children who were followed over a 2-year period. Assessing the contribution of a wide range of predictors, we show that word frequency, allomorph, lexicon size, inflectional properties of the first language (L1), and months of exposure to English all have impact on English L2 children's use of 3SG -"s" in obligatory contexts. This study enhances both our understanding of the development of 3SG -s and of child L2 acquisition. The outcomes support a usage-based approach to learning inflection and emphasize the importance of a multifactorial analysis of language development. (Contains 2 figures, 6 tables, and 2 notes.)