Lexical bundles are prefabricated fixed structures, both phrasal and clausal, which are one of the important components of fluent linguistic production. These multi-word structures are extended collocations which appear in text more frequently than expected and play an important role in constructing academic phraseology. Lexical bundles have been investigated in academic texts across the world but their variation or similarity across the academic disciplines remains an open question. The current study is a corpus-based investigation of lexical bundles in PhD dissertations in Pakistani context. The study explores 4 to 7 word lexical bundles in a corpus of 4.6 million words comprising dissertations from three different disciplines of doctoral study in Pakistani universities, i.e. English Studies, Social Sciences, and Bio-Sciences, to find out the use, variation and preferred structures across different disciplines. The lexical bundles extracted with the help of AntConc 3.4.4w (Windows) 2014 were categorized according to the structural taxonomy provided by Biber et al. (1999). The results show that all of the dissertations from three disciplines rely heavily on the prefabricated chunks. On the other hand, the common bundles show that every discipline, and even subjects within a discipline, has varied choices of lexical bundles that confirms that lexical bundles vary from subject to subject and discipline to discipline. Moreover, different disciplines have different dominant structures, for instance, English Studies have the highest percentage of Prepositional Phrase Fragments, Social Sciences have the highest percentage of Noun Phrase Fragments and Bio-Sciences have the highest percentage of Verb Phrase Fragments. The analysis shows that occurrence of 4-Word lexical bundles is almost 10 times higher than that of 5-Word bundles which means that length of structure is inversely proportional to its occurrence in the corpus. Finally, academic phraseology is not fixed, although deeply rooted in prefabricated structures in its orientation, it varies across various disciplines of study.