One of the few surviving examples of Medieval Welsh erotic verse is Cywydd y Gal, or "Poem of the Penis." The poem exists in some eleven manuscripts from the late 15th and 16th centuries, most of which are now housed in the National Library of Wales at Aberysthwyth. In seven of these, including MS. Peniarth 49, the poem is either directly or indirectly ascribed to the 14th century poet Dafydd ap Gqilym, and in the remaining four to the 15th century poetess Gweirful Mechain.1 In this paper I shall be concerned with establishing the authorship of the poem taking into consideration internal linguistic and metrical evidence.