The current study longitudinally examined a crucial individual difference variable -- i.e. awareness (operationalized as explicit attention and articulatory knowledge) -- in adult second language (L2) speech learning in the context of 40 Japanese learners' English /[alveolar approximant]/ pronunciation development in an EFL classroom. The participants' speech, elicited from word reading, sentence reading and timed picture description tasks at the beginning and end of one academic semester, were analysed in terms of three acoustic dimensions of English /[alveolar approximant]/: third formant (F3), second formant (F2) and duration. Whereas the participants showed gains in the relatively easy aspect of the English /[alveolar approximant]/ acquisition (F2 reduction for tongue retraction) as a function of increased L2 input, their explicit awareness of accurate English /[alveolar approximant]/ pronunciation played a significant role in the acquisition of the relatively difficult dimension (lengthening phonemic duration). The awareness-acquisition link was not found, however, for the most difficult dimension (F3 reduction for labial, alveolar and pharyngeal constrictors) at least within the timeframe of the project.