A study is reported in which the relationships between in-group identification and intergroup differentiation and between intergroup differentiation and self-esteem were tested in the context of political affiliation. The prediction derived from social identity theory — that there should be a positive relationship between the strength of an individual's identification with a political party and the extent to which he or she differentiates between the in-group and various political out-groups — was compared with predictions derived from realistic conflict theory and the contact hypothesis. Intergroup differentiation was measured along dimensions of evaluation, affect and perceived intragroup homogeneity. The relationships between these ratings of intergroup differentiation and the individual's self-esteem were then tested. The study was based on a survey of 199 respondents supporting five political parties in Britian. Findings showed that in-group identification and perceived material conflict were consistent predictors of intergroup differentiation along evaluative and affective dimensions but not along the dimension of perceived intragroup homogeneity. The relationship between intergroup differentiation and self-esteem proved inconsistent suggesting that some modification may be required of the central role attributed to self-esteem in social identity theory.