A longitudinal study of normal first language acquisition is being conducted by a group including linguists and students of behaviour. Babies are studied from birth and followed by weekly observations with their mothers in their own homes. Raw data consists of (1) tape-recorded sound track of baby, mother and environmental sounds, (2) concurrent dictated description of behaviour and interaction of subjects by psychiatrist-observer and (3) stop-frame film of subjects. Taped infant vocalization is studied by two linguists independently. This contribution focusses on one subject's acquisition of the word “shoe” from her first acquaintance with the word in association with the object to her capacity to use the word spontaneously with definitive pronunciation. Six stages are described and documented: Stage 1—Mother pronounces word in association with object; Stage 2—Child attempts imitation which mother rejects; Stage 3—Child's imitation accepted by mother; Stage 4—Child uses word spontaneously in association with object and is understood by mother; Stage 5—Child uses word spontaneously when object is not present; Stage 6—Child achieves adult pronunciation of word. At this point the word could be understood by anyone without context. Advantage of joint behavioural and linguistic study of speech development is stressed.