This thesis comprises an introduction, a lemmatic commentary, and indices. The introductory chapter, apart from a brief discussion of a more general nature, investigates the play and the relation it bears to Philemon's Emporos, its lost Greek model, especially with regard to the actdivisions of the Greek play and the pacing of the action in Plautus' adaptation. The commentary is provided to address problems posed by the Latin text, notably those of exegesis, textual criticism, metre, grammar, humour, imagery, staging, and the relationship to the Graeco-Roman comic tradition. An attempt is also made to distinguish between elements which may reflect the Greek comic tradition and those which suggest Plautine origin. In recent work about Plautus and Philemon it has been argued that the plot of the Emporos underwent far-reaching changes at the hands of Plautus, but the author of this thesis argues for the essential unity of the Mercator and for Plautus' conservative treatment of the plot of the Greek original, at the same time allowing for the fact that Plautus may have Romanised, exaggerated, and extended Philemon's play at certain points. By its structure, metrical arrangement, pacing, juxtaposition of contrasting types, parallel arrangement of core scenes, and the recurrence of key imagery, themes and motifs, the Mercator proves to be a carefully conceived, effectively balanced, and well-composed play.