1. Dispersal of adult stream insects may be of considerable importance in regional population dynamics and colonisation of new sites, but quantifying the rate and extent of dispersal is difficult. 2. We used stable isotope (15N) enrichment to mark more than 1.5 million larval stoneflies (Leuctra inermis) before they emerged from an upland stream in the Plynlimon area of mid-Wales, in order to determine directly the rate and pattern of inter-site dispersal. 3. A small number of isotopically enriched adult stoneflies were captured in samples taken at adjacent streams between 800 m and 1.1 km away from the source population, including a headwater of a different river system. 4. The distribution of marked individuals suggested that wind influences dispersal direction in the uplands, but the low number of captures limits our ability to draw firm conclusions. 5. This is the first direct demonstration of dispersal of insects between streams. The dispersal distances recorded were significantly greater than those suggested by previous direct studies, but much more consistent with indirect studies based on genetic differentiation of populations.