By administering a Stroop task to college-student bilinguals varied in self-rated first- (L1) and second-language (L2) proficiency, the current study examined the effects of L1 and L2 proficiencies on selective attention performance. We conducted ex-Gaussian analyses to capture the modal and positive-tail components of participants' reaction time distributions. Both L1 and L2 proficiencies were associated with a shift of reaction time distributions in incongruent trials, relative to congruent trials, and the tail size of reaction time distributions regardless of trial types. This suggests that bilinguals' L1 and L2 proficiencies could affect their Stroop performance via modulating their conflict resolution and goal maintenance abilities.