This study assessed the importance for fish of lateral connectivity of river side-channels to the River Frome, a lowland chalk. stream in Southern England. Lateral connectivity and habitat complexity is under threat in many floodplain river systems due to human disturbance. A holistic approach was used to investigate fish communities in seven sidechannels (including drainage ditches, natural streams and a millstream) and to assess the functionality provided by these habitats to fish species. Seasonal electric fishing over three years was used to monitor fish assemblages in each channel in relation to biotic and abiotic variables. Fish movements between the main channel and lateral habitats were monitored continuously in five locations with PIT (passive integrated transponder) telemetry. In total over one hundred pike (Esox lucius) and dace (Leuciscus leuciscus) were radio tracked to monitor the movements of individual fish with increased sensitivity. Each side-channel provided a distinct habitat and supported a different fish community. Flow was the main discriminating factor between channels and their assemblages. Multiple linear regression of/biotic and abiotic variables did not predict abundances of individual species effectively. In contrast, habitat stability was a good indicator of species diversity and may prove a useful management tool. Side-channels were used for different functions by different species. A single fish species used a range of habitats within the river system, each for different functions with functional differences in use between seasons. The River Frome dace population exhibited a metapopulation structure with a main river source population and sink sub-populations in side-channels. The structure of the population is based upon the availability of both lateral' and longitudinal connectivity, with dace moving between the main river and side-channels and also making excursions over 10 km along the main river channel. Drainage ditches are a particularly important pike spawning location, with males arriving earlier onto this distinct habitat. Protandry (early arrival strategy of some males) was exhibited by slow-growing male pike which arrived as early as December. Growth was also related to home range size in males, with faster growing males inhabiting larger, home ranges.