This study examined the relationship between personality and interpersonal assertiveness styles, an important and neglected topic. In all, 396 working adults completed a six-factor personality test measuring work-related traits (HPTI) and a two-dimensional assessment of interpersonal styles (III) assessing four styles: Assertiveness, Passiveness, Hostile aggression, and Manipulative aggression. We were particularly interested in the correlates of aggressive and passive behaviour, as opposed to assertive behaviour. The results suggested that those with low Conscientiousness and Adjustment (i.e. Neuroticism) but high Competitiveness (low Agreeableness) were more interpersonally aggressive, while passivity was negatively associated with all traits, particularly Adjustment, but not with Conscientiousness. Regressions indicated very different traits associated with each of the four interpersonal styles. Assertiveness was associated with sex and age, but only one trait, Risk Approach (or Courage). Limitations of these findings and implications of assessing and teaching assertiveness are discussed and considered., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.