Spanish heritage speakers have been shown to have incomplete knowledge of dative case marking with both animate direct objects (also known as differential object marking (DOM) or "a-personal") and dative experiencers with "gustar"-psych verbs in oral and written modes (Montrul, 2004; Montrul & Bowles, in press). In general, Spanish objects that are animate and specific are obligatorily marked with the preposition a ("Juan conoce "a" tu hermana" "Juan knows your sister"). Inanimate objects are unmarked ("Juan compro un perro" "Juan bought a dog", "Juan escucho la radio" "Juan listened to the radio"). "Gustar"-type psych verbs take dative experiencers obligatorily marked with the dative preposition a and a dative clitic (A "Juan le gusta el rugby" "Juan likes rugby"). This study investigated the effects of instruction on the acquisition of DOM and gustar-verbs for heritage language learners. A total of 45 2nd generation Spanish heritage speakers participated in the study, completing a pre-test, instructional treatment, and a post-test. The instructional treatment consisted of an explicit grammatical explanation of the uses of a followed by three practice exercises, for which participants received immediate, explicit feedback, including negative evidence. Results of the heritage learners' pre-test confirmed that their recognition and production of "a" with animate direct objects and dative experiencers is probabilistic, compared with a baseline group of 12 native speakers of Spanish. Post-test results revealed highly significant gains by heritage learners in both intuitions and production, suggesting that instruction, including both positive and negative evidence, facilitates classroom heritage language acquisition, at least in the short term. (Contains 1 table, 8 figures, and 3 notes.)