Dytiscid beetles are significant predators in freshwater aquatic ecosystems, playing a major role in structuring macro-invertebrate communities in some habitats (Cobbaert et al 2010). Great Diving Beetles (Dytiscus spp.) can be among the top predators, yet more than one species may be present in a particular physical location, prompting questions regarding how the Dytiscus species co-exist. This study investigated Dytiscus marginalis Linneaus 1758 and the much rarer Dytiscus dimidiatus Bergsträsser 1778 which occur together in drainage ditch ecosystems in the Somerset Levels and Moors in the United Kingdom. Estimates of niche breadths were made in relation to seasonal activity patterns, habitat usage and prey in order to gauge the degree of specialisation displayed by the two species. Findings broadly supported the view that D. marginalis is more of a generalist species than D. dimidiatus, however, a considerable degree of niche overlap was shown to exist. Evidence was found of a stronger preference in D. dimidiatus for shaded watercourses and for sections of ditch with less extensive coverage of duckweed (Lemna spp.) in the early part of the season. There were indications of both inter-specific and intra-specific predation of larvae by adults and larvae of Dytiscus spp. A major challenge overcome during the study concerned how to distinguish the larvae of the two species. Molecular ecological techniques (RAPD, PCRs and gene sequencing) were compared with morphological means to determine species identity. A relatively simple molecular method was found to distinguish the species based on species-specific sequences within a short fragment of the mitochondrial cytochrome oxidase 1 (CO1) gene. This technique successfully identified 90% of 108 individual larvae tested whereas morphology-based analysis failed to resolve them. The implications for conservation practice arising from these observations are discussed in relation to D. dimidiatus, which is considered at risk in the UK.