Formation of microRNA (miRNA) requires an RNaseIII domain-containing protein, termed DICER-1 in animals and DICER-LIKE1 (DCL1) in plants, to catalyze processing of an RNA precursor with a fold-back structure . Loss-of-function dcl1 mutants of Arabidopsis have low levels of miRNA and exhibit a range of developmental phenotypes in vegetative, reproductive, and embryonic tissues . In this paper, we show that DCL1 mRNA occurs in multiple forms, including truncated molecules that result from aberrant pre-mRNA processing. Both full-length and truncated forms accumulated to relatively low levels in plants containing a functional DCL1 gene. However, in dcl1 mutant plants, dcl1 RNA forms accumulated to levels several-fold higher than those in DCL1 plants. Elevated levels of DCL1 RNAs were also detected in miRNA-defective hen1 mutant plants and in plants expressing a virus-encoded suppressor of RNA silencing (P1/HC-Pro), which inhibits miRNA-guided degradation of target mRNAs. A miRNA (miR162) target sequence was predicted near the middle of DCL1 mRNA, and a DCL1-derived RNA with the properties of a miR162-guided cleavage product was identified and mapped. These results indicate that DCL1 mRNA is subject to negative feedback regulation through the activity of a miRNA.