The co‐occurrence of amphipods Niphargus timavi and Gammarus fossarum was studied. A small brook in SW Slovenia was sampled monthly for one year. Below the primary spring the brook sinks, flows for about 150 m below the surface and springs again. Gammarus fossarum never reached the stretch above the sink. Both species were found at all sampling sites below the secondary spring throughout the year, but the abundance of the two varied along the stream spatially and temporarily. Differences in life cycles between G. fossarum and N. timavi were not informative; however, the estimated reproductive potential of G. fossarum was much higher than of N. timavi. A result of the higher reproductive potential may be the dominance of gammarids in permanent waters, while N. timavi may invade a surrounding fissure system where it survives gammarid competition and summer desiccations when the gammarid population becomes extinct.