Water-oat Zizania latifolia (Gramineae: Oryzeae) is an asexual aquatic vegetable with an edible swelling, plump fleshy stem formed by infection of a smut fungus Ustilago esculenta to its meristems. It is widely cultivated in China because of its nutritional value and economic benefits. Rust fungus, Uromyces coronatus, is considered to be one of the most destructive pathogens causing significant yield and economic losses of Z. latifolia. The abuse of fungicide depresses the growth of Us. esculenta and causes low or no stimulated stem swell of Z. latifolia. Gall midge, Mycodiplosis uromyces (Diptera: Cecidomyiidae), feeding on the urediospores produced by Ur. coronatus is found for the first time. It has a potential value for controlling the rust pathogen prevalence, however, the biology and morphology of this species have never been documented. Herein, we investigated the morphological characteristics of the M. uromyces using optical and scanning electron microscopies, aiming to supply first-hand information, which could be informative to further investigate the ecology, behavior, genetics and utility of this species for effective biological control of Ur. coronatus.