Maine upgraded its seat belt law to primary enforcement on September 20, 2007. Belt observations during the day and night were conducted along with awareness surveys in state licensing offices. Both daytime and nighttime observed belt use increased in the months following implementation of the law (daytime 77–84%; night 69–81%). Although daytime belt use was generally higher, nighttime belt use showed a greater increase than daytime belt use. Awareness surveys indicated that Maine motorists were clearly aware of the law change and its consequences. Survey respondents also indicated having heard both messages about enforcement and messages encouraging belt use. Primary seat belt laws may have a stronger effect on belt use at night—when risk is higher—than during the day.