The California Hass avocado ( Persea americana ) is an example of a domesticated berry fruit that matures on the tree during its growing season but ripens only after being harvested. Avocados are typically harvested multiple times during the growing season in California. Previous research has demonstrated potential health benefits of avocados and extracts of avocado against inflammation and cancer cell growth, but seasonal variations in the phytochemical profile of the fruits being studied may affect the results obtained in future research. Therefore, in the present study, avocados were harvested in January, April, July, and September, 2008, from four different growing locations in California (San Luis Obispo, Ventura, Riverside, and San Diego) and analyzed for total fat content, fatty acid profile, carotenoids, and vitamin E. A significant increase in total carotenoid and fat content of avocados from all regions was noted as the season progressed from January to September. Four carotenoids not previously described in the avocado were quantified. The total content of carotenoids was highly correlated with the total fat content (r = 0.99, p < 0.001) demonstrating a remarkable degree of constancy of carotenoid intake per gram of fat content in the California Hass avocado. Future clinical research on the health benefits of the avocado should specify the time of harvest, degree of ripening, growing area, and the total phytochemical profile of the fruit or extract being studied. These steps will enable researchers to account for potential nutrient-nutrient interactions that might affect the research outcomes.