The main objective of this study is to describe the main characteristics of the Spanish interlanguage of a group of Lithuanian learners, as well as to establish differences in the evolution of the interlanguage among learners of the basic and advanced levels. Four main types of errors will be studied: morphosyntactic, lexical, phonetic and orthographic, using linguistic and statistical criteria and trying to offer as complete analysis as it is possible, both in the descriptive plan and in explaining the hypotheses about the cause of the errors. The purpose is to get data for improving the teaching methodology and for creation of specific materials for teaching Spanish, aimed at native speakers of Lithuanian.