The purpose of this study was to explore a possible season of birth effect on big five personality traits, taking into account also gender and circadian typology variables. A sample of 419 healthy young adults (249 females and 170 males) was administered the self-report version of the Big Five Observer (BFO) and the Morningness -- Eveningness Questionnaire (MEQ) to determine, respectively, personality features and circadian preference. Participants were subdivided into four season groups, according to their birth date. No significant season of birth effect was observed as regards personality traits even if summer-born participants tended to score lower on the conscientiousness dimension. In regard to the conscientiousness domain, there was a significant interaction between gender and season of birth. Males presented greater conscientiousness variation with summer born participants scoring lower. On the basis of a multiple regression analysis, MEQ score proved to be a better predictor of conscientiousness than gender, age, and season of birth. The results provide no clear evidence for a season of birth effect on the big five personality traits. It is possible that circadian preference may modulate the season of birth effect on personality traits.