A new species of gall midge, Stenodiplosis phragmicola Sinclair and Ahee (Diptera: Cecidomyiidae), is described. The host plant, Phragmites australis (Cavanilles) Trinius ex Steudel (Poaceae), is a tall, widely distributed emergent aquatic macrophyte. An introduced subspecies of the plant is considered invasive in North America (although a native subspecies also occurs). Insect specimens were collected during September 2010 and 2011 from the florets of common reed at 12 sites around Peterborough, Ontario, Canada. Preliminary data on ecological interactions between the plant and the insect are presented. Out of 2400 florets sampled, 9.5% were host to larvae of S. phragmicola. Three sites had much higher rates of infestation, with between 20% and 30% florets, and up to 100% of sampled shoots containing fly larvae. The largest stands in the sample all hosted fly populations and there was a positive association between the reproductive output of P. australis stands (measured as the average inflorescence mass per stand) and the proportion of shoots per stand containing larvae. The occurrence of S. phragmicola on the native subspecies of P. australis is also documented. Insects that consume reproductive structures of the common reed have not previously been reported from the plant's introduced range in North America.