An interesting paradox in the developmental literature has emerged in which fast-habituating babies tend to be temperamentally difficult and fast language learners, even though temperamentally difficult babies tend to be slow language learners. The purpose of the present investigation was to examine whether the paradoxical relationships among habituation, temperamental difficulty, and language acquisition could be mediated partly or wholly by infant attentional focus, because the latter also tends to correlate with temperamental difficulty and vocabulary size. Forty mother-infant dyads were followed from child age 5-20-months. Results replicated those of Tamis-LeMonda and Bornstein ("Child Develop" 1989, 60, 738-751): measures of visual habituation at 5 months were related to 13-month vocabulary. However, relationships between 5-month habituation and 20-month vocabulary were moderated by temperamental attentional focus. For children low in attentional focus, 5-month habituation was related negatively to 20-month productive vocabulary; whereas for children high in attentional focus, early habituation was positively related to later vocabulary. Results are consistent with a model of habituation in which volitional attentional focus overrides basic attentional mechanisms that occur during habituation. (Contains 5 tables.)