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Abstract: The paper presents the results of in situ observations related to Arctic clouds where possible ice-multiplication via stellar-crystal fragmentation processes is discussed. The microphysical and morphological cloud-parameters are derived from airborne measurements performed with a Cloud Particle Imager (CPI). One principal idea of this study is to demonstrate the potential of the CPI as an instrumental means to access the controversial topic of natural and instrumental crystal break-ups, and the importance of human over automated software for ice crystal classification. For the particular cloud flight of May 20th in 2004, a minimum of 18% of all observed fragmented stellar crystals have been identified as resulting from a natural fragmentation process (indication for possibly subsequent growth and/or lack of a fresh break-up line), therefore contributing to the ice-multiplication in the clouds. In other words, up to 82% of the fragmented crystals still may have experienced break-up by the CPI instrument itself. The performed simulation study for crystal break-up shows quite good agreement between the outcome of a simplest model and the experimental data. [Copyright &y& Elsevier]