This research identifies and analyzes the rhetorical structure of VHF communication in English on Merchant Marine ships and the communicative purposes underlying this communication, using SWALES (1990) model originally created for the analysis of introductions in research papers. The research also analyses the lexical choices for the genre, based on Halliday e Hasan (1976; 1994). VHF communication includes five types of basic procedures: Exchange, Broadcast, Pan Pan, Mayday, and Sécurité. The qualitative study was conducted by means of the textual analysis of 33 simulations of VHF communication, all taken from course books and practice materials for Maritime English, used at the Merchant Marine course in Rio de Janeiro. Interviews were also conducted with five student- officers from the Nautical area, so as to confirm the analysis of the variations identified in the simulations and to clarify the communicative practices of the merchant community, in light of the variations observed in the texts. The textual analysis shows obligatory and optional elements in the rhetorical structure of the simulated communications and indicates that the variations in the structure are primarily lexical. The results also show that each movement possesses its own set of functions that are realized by lexical and grammatical choices.