This study investigates diglossic patterns of language use by speakers of Zoroastrian Dari in the city of Yazd, where most of Zoroastrian population of Iran lives. Efforts have been made to find out how, when and why the spoken language of Dari is favoured by Zoroastrian community members. For this reason, the evaluation by the informants of their abilities in the majority language, i.e. Persian, their different mother tongue behaviours, motivational factors for their use of their mother tongue and domains of their mother tongue use were specifically investigated. The participants were male/female students/teachers of Zoroastrian schools and members of charitable establishments for older Zoroastrians, hence from a wide range of age groups. Information about linguistic behaviours of the target group was obtained qualitatively through interviews, diaries and participant observations in specific settings, and quantitatively through written questionnaires based on Kouzmin's work (1998), as well as several other studies analysing domains of language use in bilingual and diglossic settings. Moreover, the effects of education, sex and age in the perceptions, attitudes and speech behaviours of Zoroastrian community and the significance of differences were explored. The results indicate different sociocultural factors at work in determining the participants' choice of linguistic codes.