The main observations were carried out on imaginal populations of E. cyathigerum at Dunham Park, Cheshire. The capture-recapture method using unique marking was extensively employed, as well as direct observations. – Tenerals moved away from water during the maturation process. Most individuals marked had completed their maturation and had returned to water. Males at water were always cerulean blue with black markings, but females showed a great variation in ground colour. The form of the emergence and population curve and the long flying season strongly suggest that it is a summer species. Two population peaks, in June and July, were observed: the second peak was the larger of the two and was temporally dispersed suggesting that a large proportion of the larval population overwintered in the antepenultimate or younger instars. The Enallagma population at Pond 3 was almost extinct in June 1970, whereas a total estimated population of about 500 was present in the 1966 season. The highest steady density counts ranged from 16.8 to 47.8 insects per 100 metres of shore line at Dunham, but an exceptional value of 363 per 100 metres was recorded at Pen Ponds, Surrey, in 1967, It is suggested that physiological condition is more important in controlling numbers at water than male intra-specific interactions. Post-maturation mortality is probably nearly random with respect to age, but the marking process seemed to result in an abnormally high rate of egress in the first day after initial capture. The mean length of life for males at water was about 12 days and the maximum observed survival was 39 days. Most male individuals of E. cyathigerum show a greater mobility within the colony area than males of Ischnura elegans.