Second language learners show various forms of mispronunciation, or modification, of target pronunciation, most perhaps due to direct native language transfer, but others, summarized here, to deflected contrast, hypercorrection and covert contrast. The present paper reports on a novel form of adaptation that is termed ‘quasi-neutralization,’ in which acoustic characteristics of competing target phonemes are found within the same interlanguage segment (e.g., think [θɪŋk] pronounced as [θsiŋk]). The three English voiceless coronal fricatives /s/, /ʃ/, /θ/ were elicited from Japanese learners of English via two techniques: a wordlist reading task that encouraged participants to focus on their pronunciation, and a sentence construction task that diverted their attention from pronunciation. Among different types of modification, quasi-neutralization was observed predominantly when participants were conscious of their pronunciation, which could reflect their linguistic insecurity as learners. This research thus illuminates another of the strategies that learners employ in the acquisition of L2 pronunciation.