The focus of this graduate thesis is the differences between phraseological units of American and British English prison slang. For the purpose of this research a corpus of 167 phraseological units was collected consisting of 60 American English and 107 British English prison slang examples. The theoretical foundation for this research and classification of PUs was provided by Sabine Fiedler’s book English Phraseology: A Coursebook. According to that author PUs can belong to conventional types, special types, and types with special elements. The greatest number of PUs in both groups belonged to phraseological nominations. There were 12 PUs belonging to rhyming slang found in British English prison slang, while there was none found in the American English corpus. Furthermore, PUs containing proper names were more used in British English compared to American English examples. In American prisons PUs with body part designations are slightly more common than in British ones. A significant number of PUs used to denote various nationalities was found in British English compared to American English PUs. On the other hand, there were more examples of PUs used for food in American examples compared to British ones. The general conclusion would be that the main difference between the two is that British English prison slang uses more coded PUs and is thus harder to understand for an individual not belonging to that language community.