A mass occurrence of the millipede Calyptophyllum longiventre (Verhoeff, 1941) in Mardin, south-east Turkey, is reported. Descriptive notes on this poorly known species are given, including the first description of male characters. Comparison is made with other instances of millipede mass occurrences, previous swarming events of millipedes in Turkey are summarized, and a note on the distribution of the genus Calyptophyllum is given.