Research attempting to understand the intermediate stages of first-language acquisition and disordered speech has led to the discovery of covert contrast. A covert contrast is a statistically reliable difference between phonemes that is produced by a language learner, but in a way that cannot be heard readily by a listener of the target language. In the present study, we aimed to extend the investigation of covert contrasts to the domain of second-language acquisition. In particular, we used ultrasound tongue imaging to examine whether adult second-language learners of American English produced articulatory distinctions between the target vowels, in addition to producing acoustic differences. We collected speech recordings from 21 speakers (7 Korean speakers, 7 Spanish speakers, 7 English controls) while they produced words illustrating the relevant vowel contrasts in English: /i/-/?/ and /[epsilon]/-/ae/. Results showed that approximately 36% of our second-language participants implemented various patterns of covert contrast in vowel articulation. Ultrasound measurements revealed that two participants made a covert distinction between two vowels that were perceptually neutralized. For one participant, the anterior part of the tongue was higher for /i/ than /?/, and for the other, it was higher for /[epsilon]/ than /ae/. Overall, our findings highlight the importance of a learner-oriented approach in studying the acquisition of phonemic contrasts and suggest that ultrasound tongue imaging can be a promising tool to examine the articulatory details of vowel production in second-language learners.