This study reports a replication of an earlier study by Kilbourne and Ginsberg (1980) which indicated the occurrence of a transition from predominantly coacting to predominantly alternating infant-mother vocalization patterns. In addition, the present study examined the modulating influences of nursing activity and mother's focus of attention upon the developmental emergence, if any, of vocal alternation. Coaction was operationalized as the occurrence, within each session, of overlapping infant-mother vocalizations. Alternation was operationalized as infant vocalization(s) occurring between the offset of the mother's initial vocalization and the onset of her next vocalization without overlap with her infant's vocalization(s). One normal male infant was observed from 10 days of age until the 22nd week of life. A 3/4 inch color television camera was employed to tape a routine cycle of activity, usually on a semi-monthly basis. Each session taped consisted of 20 minutes of filming of both play and feeding periods. Upon completion of the observation period, the video tapes were replayed and viewed by one trained observer. An Esterline Angus Event Recorder, with electric pens that were operated by finger switches, was used to code onsets and offsets of vocalizations, as well as duration of (1) all infant and mother vocalizations; (2) feeding activities; and (3) mother's attentional focus (i.e., whether she was attending primarily to her baby). In general, results from the present study replicated the results obtained in the earlier study. In both studies the performance of alternation emerged during the fourth month, suggesting a clear and apparently stable change from a predominantly coacting pattern of infant-mother vocal interactions to a predominantly alternating pattern. This stable change to alternation was not found to be an artifact of nursing activity or the mother's attentional focus. (Author/MP)