We may distinguish an ophthalmomyiasis externa (fly larvae in the outer eye) and an ophthalmomyiasis interna (entry of fly larvae into the eye). While the first usually takes a mild course (conjunctivitis) the latter nearly always leads to loss of the sight. Besides three cases of O. externa the mechanical irritating effect of the larva Oestrus ovis Lin. (stage I) on the basis of scanning electron microscopic pictures is discussed.