Skeletal muscle was examined in zebrafish larvae, to address questions related to the function of the intermediate filament protein desmin and its role in the pathogenesis of human desminopathy. A novel approach including mechanical and structural studies of skeletal muscles from 4-6 day larvae was applied. Gut motility was studied with confocal imaging. Two desmin genes were identified in the larvae with the experession both in skeletal musclese and gut. Morpholino antisense oligonucleotides were utilized to knock down desmin. Expression was assessed using mRNA and protein analyses. Histology and synchrotron-light based small angle x-ray diffraction were applied. Functional properties were analyzed with in vivo studies of swimming behavior and with in vitro mechanical examinations of muscle. The two desmin genes normally expressed in zebrafish could be knocked down by about 50%. This resulted in a phenotype with disorganized skeletal muscles with altered attachments to the myosepta. The knock-down larvae were smaller and had diminished swimming activity. Active tension was lowered and muscles were less vulnerable to acute stretch-induced injury. X-ray diffraction revealed wider interfilament spacing. In conclusion, desmin intermediate filaments are required for normal active force generation and affect the vulnerability during eccentric work in skeletal muscles. This is related to the role of desmin in anchoring contractile units for optimal force transmission. The results also show that partial lack of desmin, without protein aggregates, is sufficient for causing skeletal muscle pathology resembling that in human desminopathy.