Similarities and differences in language acquisition and variations in patterns of language dominance were studied with three four-year-old girls. The ideoqraphic study of child speech involved a monolingual English-speaking child, a monolingual Spanish-speaking child, and a bilingual English-Spanish-speaking child. The following measures were used: auditory comprehension, syntax and morphology, and temporal analysis (speech rate, pauses, and hesitation phenomena). Formal testing supplemented observation of children's speech in natural settings. Testing sessions were conducted approximately every 6 months and when the bilingual child had extended exposure to either language. Data were collected in the United States and in Mexico. Certain similarities have been found in monolingual and bilingual development. Bilingual development is both enriched and affected negatively by the constant exposure to two languages: enrichment takes place in vocabulary and facility with language, while morphology and syntax are affected negatively (in both languages) by the interference of one language system with the other. (SW)