Among the differences that distinguish colloquial Persian from the formal variety is the deletion of sounds. This study is based on a corpus of 20 minutes of conversation with three native speakers from Tehran. The results show that 6% of sounds are deleted in colloquial Persian. Consonants are more likely to be deleted than vowels. Among consonants, the most frequent are the dentals and their deletion occur at the end of the syllable, especially in consonant clusters with the same place of articulation. The deletion affects more grammatical words than lexical ones.