This study analyzes the role of Transfer, or native language interference in the written interlanguage of fourteen English-speaking students during the second semester of the third year of Spanish at Rice University. English language interference is studied in the development of Spanish lexicon, syntax, morphology and orthography. Errors from translation exercises as well as from written compositions were analyzed and categorized by type. A list of all the English transfer errors found in this project is preceded by an explanation of the interference mechanism involved. At two data collection points, evenly spaced toward the beginning and the end of the semester, a horizontal comparison of transfer errors was carried out in order to observe changes in transfer errors. A review of previous literature in language transfer precedes the findings specific to this study.