Incidental grammar acquisition involves learners "picking up" a grammatical feature while their primary focus is on some other aspect of language--either message content or another language feature that is taught directly. This article reports a study of children's incidental grammar acquisition of two grammatical features--plural "-s' and copula "be"--in two types of instruction--focus on form (FonF) and focus on forms (FonFs). The two features were not directly taught, but opportunities for learning them occurred in classroom interactions. Thirty young beginner Japanese learners were divided into two groups (FonF and FonFs) and received nine repeated lessons over 5 weeks. The study examined learners' acquisition of the two structures as measured by tests and sought explanations for the results in terms of the differences in interactions that arose in the two instructional contexts and, in particular, opportunities for attending to the two grammatical features in these interactions. The children in the FonF classroom demonstrated acquisition of plural "-s" but not of copula "be." Neither structure was acquired by the children in the FonFs classroom. Analysis of the classroom interactions show that there was a functional need to attend to plural "-s" (but not copula "be") only in the FonF classroom.