The English and Spanish utterances of 19 bilingual preschool children were monitored as they conversed with experimenters. Characteristics of the children's language are reported for a bilingual prestage (exhibited by the youngest subject) and for four developmental stages in English and Spanish. These stages are characterized by approximately parallel growth in horizontal (structural) and vertical (functional) development in both languages; specific instances of conformity and nonconformity to this rule of parallelism are noted. The ability to differentiate between the two languages developed at about the age of three years. In addition to reporting research results, this report discusses the problems encountered in selecting subjects. (JB)