The coenagrionid C. tenellum was studied by capture-recapture and direct observation in several neighbouring areas along a small stream in the New Forest, Hampshire, England. The New Forest is important ecologically because it is probably the last remaining area of heathland in lowland Britain which is large enough to support many of the characteristic plants and animals. Notably, the Forest’s bogs and nearby heathland provide habitats for C. lenellum, resulting in this species being commoner there than anywhere else in Britain. The imaginal C. lenellum was very selective in the areas of stream colonised: it was only common where the stream was wide and boggy with a dense growth of Hypericum elodes. Deeply incised and clear, gravelly parts of the stream were avoided. Most individuals in well established populations were highly static, but a high proportion of insects in sites away from the main population centres were wanderers. Most movement within an established colony was from the periphery inwards; relatively few individuals were observed to move from the centre outwards. Adult male C. lenellum show relatively little tendency to move far once they are associated with a colonial group. The population numbers declined consistently throughout the study periods (first three weeks of August, 1974 and 1975), and only five individuals out of 704 males seen were teneral. The mean length of the life for adult males in different populations varied between 8 and 17 days. Survivorship in males was nearly random with age for the major part of the life span. A detailed analysis of the capture-recapture data revealed only one occasion when marking had a deleterious effect on survival. The percentage of males ranged from 57 to 68 percent and the mean mating frequencies were 13 percent (males) and 35 percent (females). Frequent aerial fighting between males was not observed to lead directly to movement of any individuals away from the colony area. A few male-to-male interactions were definitely sexual. Mating occurred at water, in fine weather, and falls into the copulation type classified as ’long’.