This dissertation deals with interference in Batak Toba language (BT) related to the language attitudes of bilingual BT speakers living in Medan.BT language is interferenced due to the intervention of the element of Bahasa Indonesia (BI) system that there is a deviation in standard BT. The deviation is clearly revealed in the phonological, grammatical,and lexical levels.Theinterference in this language is related to the language attitudes of bilingual BT speakers.\n The purposes of this study are to a) to describe interferences found in BT, b) to describe the language attitudes of BT speakers based on the variables of sex,age,language use,and length of stay,c) to describe the relationship between the language attitudes of BT speakers and interference, and d) to describe the current use of BT in Medan.\n The main theories are used in this dissertation such as a) the languages in contact theory by Weinreich (1968) describing that interference in the relocation of language element into the other languages and the deviation of the use of rules and norms of language, b) language attitude by Anderson (1974) arguing that attitude is a belief system related to the language which lasts relatively long about a language object which makes someone tend to act in a certain way he/she likes.Garvin and Mathiot (1968) argued that there are three characteristics of language attitude such as language loyalty, language pride, and the awareness of language norms. The application of structural theory of this study is to discuss the comparison of BT – BI systems.\n\t This study employed qualitative and quantitative methods. The data for this study were collected by a passive participatory observation technique, questionnaire, and test as well as recording technique. The speech interference data were analyzed through comparative descriptive techniques, while the data of language attitude were statistically tested through t-test and ANOVA test. The statistic result of the speakers language attitude were correlated with the result of the test of interference in BT by using the Product Moment by Pearson.\n\t The result of the study showed that in Medan BT has been interferenced by BI in the phonological aspect in the forms of phoneme alteration and assimilation, morphological interference in the forming of noun and verb, interference in the aspect of syntaxe on the use of particles ni, na,on the marker of topic sentence do, ma, pe, dope, and be, and phrase construction pattern. Interference of the lexical aspect is found in noun, verb, adjective, and adverb. The result of the language attitudes of BT speakers in Medan showed a positive attitude toward BT.The relationship between language attitude with the interference of BT speakers showed a significant negative relationship which means that if the attitudes of BT speakers are more increased,the phenomenon of interference in BT will be decreasing.