OBJECTIVE The use of helmets that do not comply with safety standards is common in California. The objective of this study was to describe the use of these nonstandard helmets among San Francisco Bay-area (SFBA) motorcyclists and to identify personal and motorcycle characteristics that are associated with the use of nonstandard helmets. METHODS A survey of 860 SFBA motorcyclists was conducted. Log-binomial regression was used to estimate risk ratios to compare probabilities of nonstandard helmet use. RESULTS Fifteen percent of motorcyclists reported wearing a nonstandard helmet sometimes or often. BMW riders had the lowest use of nonstandard helmet (5%) and Harley-Davidson riders had the highest use (51%). Among non-Harley-Davidsons, riders of cruiser-style motorcycles were 3.1 times as likely to wear a nonstandard helmet as riders of motorcycles of other styles. African American riders were more than twice as likely to use nonstandard helmets compared to riders with other self-reported race. CONCLUSIONS Behavioral countermeasures are needed to improve motorcycle helmet choice in California. This study identified riders of Harley-Davidsons and riders of cruiser-style motorcycles of other brands as potential targets of interventions.