The purpose of this quantitative causal-comparative study was to determine the frequency of marijuana use by California university students based on gender and ethnicity. This study was important because marijuana is the most used illicit substance in the United States, and there is limited research regarding the demographic characteristics of California university students who use marijuana. The quantitative research design was used to describe the patterns of marijuana use by California university students. The setting was online. The participants included a random sample of 167 college students from five California state universities. The Daily Sessions, Frequency, Age of Onset, and Quantity of Cannabis Use Inventory was used to determine the frequency of marijuana use. An independent samples t test was used to determine if there were differences in the frequency of marijuana use based on gender, while a one-way analysis of variance was used to determine the differences in the frequency of use based on ethnicity. Caucasian males and females were in the highest use group, Asian males and females and African American females were in the mid-range use group, and Hispanic males and females were in the lowest range use group. The conclusion was California university students' frequency of marijuana use differs based on gender and ethnicity. Recommendations for further study include increasing the number of participants and institutions of higher education, the use of trend data, and the use of a qualitative research design. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]