Upon seed imbibition, abscisic acid (ABA) levels decrease to allow embryos to germinate and develop into seedlings. However, under abiotic stress conditions, ABA levels remain high, and growth and development are arrested. Several transcription factors, including abscisic acid-insensitive (ABI)3 and ABI5, are known to control this developmental checkpoint. Here, we show that, in germinatingArabidopsis thalianaseeds, ABA induces the accumulation of microRNA 159 (miR159) in an ABI3-dependent fashion, and miRNA159 mediates cleavage ofMYB101andMYB33transcriptsin vitroandin vivo. The two MYB transcription factors function as positive regulators of ABA responses, as null mutants ofmyb33andmyb101show hyposensitivity to the hormone. Consistent with this, miR159 over-expression suppressesMYB33andMYB101transcript levels and renders plants hyposensitive to ABA, whereas transgenic plants over-expressing cleavage-resistant forms ofMYB33andMYB101are hypersensitive, as are plants over-expressing the Turnip mosaic virus (TuMV) P1/HC-Pro viral protein that is known to inhibit miRNA function. Our results suggest that ABA-induced accumulation of miR159 is a homeostatic mechanism to directMYB33andMYB101transcript degradation to desensitize hormone signaling during seedling stress responses.