This study is a pilot investigation of how adult listeners spontaneously perceive the vowel quality in infant utterances. Recordings of two Japanese infants from the Tokyo area were used to generate the stimuli. One infant was recorded every week between 17 to 24 weeks of age. The other infant was recorded twice, at 32 and 78 weeks of age. The sessions lasted for about 1 h and included playing alone, playing with the mother, and eating. The stimuli for the perceptual evaluation were a random sample consisting of 57 utterances, drawn from the pool of noise and interference‐free utterances. Four phonetically trained subjects judged tongue height and frontness in a speeded judgment test paradigm. The results are compatible with the data from earlier acoustic studies [Davis and MacNeilage, J. Speech Hear. Res.33, 16–27 (1990)], using careful phonetic transcription. Japanese infants explore the opening dimension during the earlier period of babbling. Although the data cannot be used to establish the exact point at which front/back differentiation starts to occur, a significant difference was observed between the use of that dimension at 32 and at 78 weeks of age. [Research supported by The Bank of Sweden Tercentenary Foundation, Grant 90‐0150.]