We report the ultrastructural feature of a vitreous membrane with multiple fenestrations in a case of a 40-year-old woman with a Stickler syndrome. The left eye showed a type 2 vitreous phenotype. Vitreous abnormalities of the right eye may correspond to phenotypic conversion to a type 1 appearance of a type 2 vitreous phenotype. Fenestrated membrane may correspond to posterior hyaloid membrane in complete posterior vitreous detachment in this phenotypic conversion. Fenestrated membrane consisted of avascular fibrocellular tissue with cells arranged in a cohesive pattern around the fenestration. Ultrastructural findings of cells were characteristic of proliferative Müller cells. Ultrastructural examination of collagen fibrils showed them to be similar to normal vitreous. This finding suggests that collagen molecules are not functionally altered and are probably quantitatively insufficient during vitreous development.