The 'Fazienda de Ultramar' is considered to be the earliest extensive prose work in Castilian. The aim of this study is to assess, quantify and examine in detail the linguistic variation in the text and attempt to explain this variation with reference to the various factors that may influence it: internal linguistic factors such as palaeographic, morphological and syntactic considerations, or external extra-linguistic factors comprising variatio, register and scribal considerations. I focus on six variables. Three are orthographic: the use of < m >, < n >, or < ˉ > to represent /M/ before a bilabial; the use of < i > or <y> to represent /i/; the use of < l > to represent /ʎ/ and < r > to represent /r/ intervocalically. Three are morphological variables: variant forms for derivatives of Latin quōmŏdo; -ie and -ia Imperfect and Conditional forms; weak object pronoun apocope. Amongst the most significant conclusions from this study are the following: I demonstrate that the extent of leísmo in the Fazienda is much greater than that proposed by Echenique (1981). On the basis of my analysis of the data from the Fazienda, I argue that the extent of leísmo in other medieval Castilian texts needs to be re-assessed. The information provided by CORDE on the use of cuemo and cumo pre-1250 shows a concentration of these forms in documents from Northern Castile. The discovery of the phrase por consieglo, previously unique to the Fazienda, in one of these documents also establishes a further connection to Northern Castile. I suggest that the manuscript may well originate from this region. It is generally accepted that more than one scribe was involved in the copying of the Fazienda. I test the hypothesis that different scribal interventions may account for some of the variation in the Fazienda and find evidence to support this hypothesis.