Recent research indicates that people respond socially to computers and perceive them as having personalities. Software agents are artifacts that particularly embody those qualities most likely to elicit social responses: fulfilling a social role, using language, and exhibiting contingent behavior. People's disposition to respond socially can be so strong that they may perceive software agents as having a personality, even when none was intended. The following is a discussion about intentionally designing personalities for social agents. To design personalities, it is necessary to consider the nature of personality and its role in interactions between people and artifacts. In addition, a case study of designing a social software agent is presented. The conclusions from this experience are summarized as guidelines for future agent developers.