Published records and data from the California Waterfowl Association’s program monitoring nest boxes show that the Hooded Merganser (Lophodytes cucullatus) has become a regular and widespread breeder in California. More than 100 recorded nestings between 1996 and 2004 were distributed over 20 counties and yielded more than 800 hatchlings. Nest-box data show a dramatic increase in successful nesting attempts by the Hooded Merganser over this period. Banding data show a high degree of nest-site fidelity among females. Christmas Bird Count data reveal a statistically significant increase in the winter abundance of the Hooded Merganser in California since the early 1980s. This expansion of the breeding range is probably due to some combination of increasing breeding habitat and nest boxes and an increase in the species’ population in the western portion of its range.