The timing of when to initiate reproduction is an important transition in any organism's life cycle. There is much variation in flowering time among populations, but we do not know to what degree this variation contributes to local adaptation. Here we use a reciprocal transplant experiment to examine the presence of divergent natural selection for flowering time and local adaptation between two distinct populations of Mimulus guttatus. We plant both parents and hybrids (to tease apart differences in suites of associated parental traits) between these two populations into each of the two native environments and measure floral, vegetative, life-history, and fitness characters to assess which traits are under selection at each site. Analysis of fitness components indicates that each of these plant populations is locally adapted. We obtain striking evidence for divergent natural selection on date of first flower production at these two sites. Early flowering is favored at the montane site, which is inhabited by annual plants and characterized by dry soils in midsummer, whereas intermediate (though later) flowering dates are selectively favored at the temperate coastal site, which is inhabited by perennial plants and is almost continually moist. Divergent selection on flowering time contributes to local adaptation between these two populations of M. guttatus, suggesting that genetic differentiation in the timing of reproduction may also serve as a partial reproductive isolating barrier to gene flow among populations.