After a brief consideration of two perennial theories that have influenced research on bilingualism in American schools, this paper focuses on recent research using the Berko method to study acquisition of the English plural by Spanish‐English bilinguals in grades 1 through 10, in San Antonio, Texas. Method and theory in two highly similar studies (Natalicio 1969, and Johnson 1973) are contrasted in detail in an attempt to explain the differences between the two studies' conclusions. Of special interest in the Johnson study are the differing statistical validities found in the same data evaluated by differing criteria, and the elicitation of some data that suggests that the Berko test is measuring a type of linguistic ability other than what it is often assumed to measur