Hydrellia personata Deonier (Diptera: Ephydridae) is a rarely collected shore fly found in the western and midwestern United States. We encountered a population in a constructed wetlands in southern California where the larvae were miners of duckweed (Lemna minor L.). Eggs were inserted between the upper and lower layers of epithelial tissue. Newly hatched larvae either bored into a duckweed thallus or separated the upper and lower epithelial layers to gain access to the photosynthetic tissue. Each instar mined several plants prior to molting inside of a plant thallus, and pupariation occurred inside a hollowed-out thallus. Several pupae were parasitized by the braconid [A Chalcidoid!!] Cyrtogaster clavicornis Walker. Overall, the biology and morphology of the immature stages were very similar to that of the closely related genus Lemnaphila. We describe all immature stages and redescribe the adult (male and female) based upon the newly collected materials. The unusual adult morphology of this species prevents its placement into any of the existing species groups of Hydrellia. none