The timing of the transition to flowering is critical for reproductive success in plants. Arabidopsis FCA encodes an RNA-binding protein that promotes flowering. FCA expression is regulated through alternative processing of its pre-mRNA. We demonstrate here that FCA negatively regulates its own expression by ultimately promoting cleavage and polyadenylation within intron 3. This causes the production of a truncated, inactive transcript at the expense of the full-length FCA mRNA, thus limiting the expression of active FCA protein. We show that this negative autoregulation is under developmental control and requires the FCA WW protein interaction domain. Removal of introns from FCA bypasses the autoregulation, and the resulting increased levels of FCA protein overcomes the repression of flowering normally conferred through the up-regulation of FLC by active FRI alleles. The negative autoregulation of FCA may therefore have evolved to limit FCA activity and hence control flowering time.