Identity-related accounts of language alternation among bilingual speakers have traditionally drawn on the social values of the languages involved in specific communities (Gumperz 1982, Myers-Scotton 1993). However, recently researchers have expressed reservation against this approach (Wootton and Sebba 1998, Li Wei 1998, 2002). Following from this, Gafaranga 2001 argues that, in order to account for the orderliness of language alternation, language preference (Auer 1984) must be seen as a membership categorization device (Sacks 1966, 1974). In developing his argument, Gafaranga draws on instances of ordinary everyday conversation among bilingual speakers. In this paper, we take Gafaranga's argument a step further and look at language alternation in first-time trilingual service encounters collected in the Barcelona area. The languages involved are Catalan, Castilian, and English. Analysis of these data reveals that, in addition to doing service-relevant tasks, participants accomplish “medium-related activities” (Gafaranga 2001) drawing on their various linguistic identities. Therefore, the analysis confirms the need to see linguistic identities as social identities in their own right.