Dissertation (MA Public Administration)The study aimed at evaluating the effectiveness of the field administration in controlling illegal immigrants in Tanzania. It is a sort of case study carried out in Biharamulo District. A special attention was made examining the methods used by field administration to prohibit illegal migrants in Tanzania. Unraveled also were the challenges of field administration in curbing illegal immigration in Tanzania. The study was a descriptive design and a sample of 60 respondents was used and these were randomly obtained from Biharamulo District. The methods used in data collection were interviews, questionnaires, documentary review and focus group discussion. The statistical package for social science (SPSS) was used for management of data and analysis. In this study the theories pertaining to migration were used to familiarize the reader with paradigms, frameworks and approaches in the field. Data was subjected to frequency counts involving simple percentages and content analysis techniques using themes and comments from questionnaires and interview. This was to address the four research objectives formulated to guide the study. The study reveals that the methods used by field administration to prohibit illegal migrants in Tanzania includes, identifying them in the society, informing the authority, educating the people about the side effect of the illegal migrants and through collaboration with the security institutions like police. The study recommends that, regular sensitization should be organized in the country especially those places bordering neighboring countries. Most of the neighboring countries are facing political instabilities which is the mother cause of the illegal migration in our country. The issues of Rwanda and Burundi genocide and the problem of FNL and Lords Resistant Army (LRA) in Uganda contribute a lot in the issue of illegal migration in Tanzania. Apart from that the issue of Ali shabab in Somalia also contributes in the problem of illegal migration in the country, other things like famine in Southern Sudan and its political instability further strengthens this problem to our country.