Critical swimming speeds (U_crit) and morphological characters were compared between the F₄ generation of GH-transgenic common carp Cyprinus carpio and the non-transgenic controls. Transgenic fish displayed a mean absolute U_crit value 22·3% lower than the controls. Principal component analysis identified variations in body shape, with transgenic fish having significantly deeper head, longer caudal length of the dorsal region, longer standard length (L_S) and shallower body and caudal region, and shorter caudal length of the ventral region. Swimming speeds were related to the combination of deeper body and caudal region, longer caudal length of the ventral region, shallower head depth, shorter caudal length of dorsal region and L_S. These findings suggest that morphological variations which are poorly suited to produce maximum thrust and minimum drag in GH-transgenic C. carpio may be responsible for their lower swimming abilities in comparison with non-transgenic controls.