Although learning of phonological word forms is important for mastering a language, little is known about the factors influencing it. We addressed this question by comparing phonological word form learning in two situations: learning novel word forms as the labels for referents (deliberate word learning); and learning novel word forms through incidental exposures to the word forms alone, without any referents (incidental word form learning). Phonological word form learning as measured by stem completion ability was found to be better in the former than in the latter situation (Experiment 1). Experiment 2 found that deliberate memorization of word forms, even without any referents, also yielded better stem completion ability than purely incidental learning. These results suggest that incidental word form learning may not yield full mastery of word forms, and that deliberate learning may be a necessary component for such mastery.